Provided by: units_2.23-1build2_amd64 
NAME
units — unit conversion and calculation program
SYNOPSIS
units [from-unit [to-unit]]
units [-hcemnSpqsv1trUVI] [-d digits] [-f units file] [-L logfile] [-l locale] [-o format]
[-u unit system]
[from-unit [to-unit]]
units [--help] [--check] [--check-verbose] [--verbose-check] [--digits digits] [--exponential]
[--file units file] [--log logfile] [--locale locale] [--minus] [--oldstar] [--newstar] [--nolists]
[--show-factor] [--conformable] [--output-format format] [--product] [--quiet] [--silent]
[--strict] [--verbose] [--compact] [--one-line] [--terse] [--round] [--unitsfile]
[--units units system] [--version] [--info]
[from-unit [to-unit]]
DESCRIPTION
The units program converts quantities expressed in various systems of measurement to their equivalents in
other systems of measurement. Like many similar programs, it can handle multiplicative scale changes.
It can also handle nonlinear conversions such as Fahrenheit to Celsius; see Temperature Conversions. The
program can also perform conversions from and to sums of units, such as converting between meters and
feet plus inches.
But Fahrenheit to Celsius is linear, you insist. Not so. A transformation T is linear if
T(x + y) = T(x) + T(y) and this fails for T(x) = ax + b. This transformation is affine, but not linear—
see https://en.wikipedia.org/wiki/Linear_map.
Basic operation is simple: you enter the units that you want to convert from and the units that you want
to convert to. You can use the program interactively with prompts, or you can use it from the command
line.
Beyond simple unit conversions, units can be used as a general-purpose scientific calculator that keeps
track of units in its calculations. You can form arbitrary complex mathematical expressions of
dimensions including sums, products, quotients, powers, and even roots of dimensions. Thus you can
ensure accuracy and dimensional consistency when working with long expressions that involve many
different units that may combine in complex ways; for an illustration, see Complicated Unit Expressions.
The units are defined in several external data files. You can use the extensive data files that come
with the program, or you can provide your own data file to suit your needs. You can also use your own
data file to supplement the standard data files.
You can change the default behavior of units with various options given on the command line. See Invoking
Units for a description of the available options.
INTERACTING WITH UNITS
To invoke units for interactive use, type units at your shell prompt. The program will print something
like this:
Currency exchange rates from FloatRates (USD base) on 2023-07-08
3612 units, 109 prefixes, 122 nonlinear units
You have:
At the ‘You have:’ prompt, type the quantity and units that you are converting from. For example, if you
want to convert ten meters to feet, type 10 meters. Next, units will print ‘You want:’. You should type
the units you want to convert to. To convert to feet, you would type feet. If the readline library was
compiled in, then tab will complete unit names. See Readline Support for more information about readline.
To quit the program type quit or exit at either prompt.
The result will be displayed in two ways. The first line of output, which is marked with a ‘*’ to
indicate multiplication, gives the result of the conversion you have asked for. The second line of
output, which is marked with a ‘/’ to indicate division, gives the inverse of the conversion factor. If
you convert 10 meters to feet, units will print
* 32.808399
/ 0.03048
which tells you that 10 meters equals about 32.8 feet. The second number gives the conversion in the
opposite direction. In this case, it tells you that 1 foot is equal to about 0.03 dekameters since the
dekameter is 10 meters. It also tells you that 1/32.8 is about 0.03.
The units program prints the inverse because sometimes it is a more convenient number. In the example
above, for example, the inverse value is an exact conversion: a foot is exactly 0.03048 dekameters. But
the number given the other direction is inexact.
If you convert grains to pounds, you will see the following:
You have: grains
You want: pounds
* 0.00014285714
/ 7000
From the second line of the output, you can immediately see that a grain is equal to a seven thousandth
of a pound. This is not so obvious from the first line of the output. If you find the output format
confusing, try using the ‘--verbose’ option:
You have: grain
You want: aeginamina
grain = 0.00010416667 aeginamina
grain = (1 / 9600) aeginamina
If you request a conversion between units that measure reciprocal dimensions, then units will display the
conversion results with an extra note indicating that reciprocal conversion has been done:
You have: 6 ohms
You want: siemens
reciprocal conversion
* 0.16666667
/ 6
Reciprocal conversion can be suppressed by using the ‘--strict’ option. As usual, use the ‘--verbose’
option to get more comprehensible output:
You have: tex
You want: typp
reciprocal conversion
1 / tex = 496.05465 typp
1 / tex = (1 / 0.0020159069) typp
You have: 20 mph
You want: sec/mile
reciprocal conversion
1 / 20 mph = 180 sec/mile
1 / 20 mph = (1 / 0.0055555556) sec/mile
If you enter incompatible unit types, the units program will print a message indicating that the units
are not conformable and it will display the reduced form for each unit:
You have: ergs/hour
You want: fathoms kg^2 / day
conformability error
2.7777778e-11 kg m^2 / sec^3
2.1166667e-05 kg^2 m / sec
If you only want to find the reduced form or definition of a unit, simply press Enter at the ‘You want:’
prompt. Here is an example:
You have: jansky
You want:
Definition: fluxunit = 1e-26 W/m^2 Hz = 1e-26 kg / s^2
The output from units indicates that the jansky is defined to be equal to a fluxunit which in turn is
defined to be a certain combination of watts, meters, and hertz. The fully reduced (and in this case
somewhat more cryptic) form appears on the far right. If the ultimate definition and the fully reduced
form are identical, the latter is not shown:
You have: B
You want:
Definition: byte = 8 bit
The fully reduced form is shown if it and the ultimate definition are equivalent but not identical:
You have: N
You want:
Definition: newton = kg m / s^2 = 1 kg m / s^2
Some named units are treated as dimensionless in some situations. These units include the radian and
steradian. These units will be treated as equal to 1 in units conversions. Power is equal to torque
times angular velocity. This conversion can only be performed if the radian is dimensionless.
You have: (14 ft lbf) (12 radians/sec)
You want: watts
* 227.77742
/ 0.0043902509
It is also possible to compute roots and other non-integer powers of dimensionless units; this allows
computations such as the altitude of geosynchronous orbit:
You have: cuberoot(G earthmass / (circle/siderealday)^2) - earthradius
You want: miles
* 22243.267
/ 4.4957425e-05
Named dimensionless units are not treated as dimensionless in other contexts. They cannot be used as
exponents so for example, ‘meter^radian’ is forbidden.
If you want a list of options you can type ? at the ‘You want:’ prompt. The program will display a list
of named units that are conformable with the unit that you entered at the ‘You have:’ prompt above.
Conformable unit combinations will not appear on this list.
Typing help at either prompt displays a short help message. You can also type help followed by a unit
name. This will invoke a pager on the units data base at the point where that unit is defined. You can
read the definition and comments that may give more details or historical information about the unit. If
your pager allows, you may want to scroll backwards, e.g. with ‘b’, because sometimes a longer comment
about a unit or group of units will appear before the definition. You can generally quit out of the
pager by pressing ‘q’.
Typing search text will display a list of all of the units whose names contain text as a substring along
with their definitions. This may help in the case where you aren't sure of the right unit name.
USING UNITS NON-INTERACTIVELY
The units program can perform units conversions non-interactively from the command line. To do this,
type the command, type the original unit expression, and type the new units you want. If a units
expression contains non-alphanumeric characters, you may need to protect it from interpretation by the
shell using single or double quote characters.
If you type
units "2 liters" quarts
then units will print
* 2.1133764
/ 0.47317647
and then exit. The output tells you that 2 liters is about 2.1 quarts, or alternatively that a quart is
about 0.47 times 2 liters.
units does not require a space between a numerical value and the unit, so the previous example can be
given as
units 2liters quarts
to avoid having to quote the first argument.
If the conversion is successful, units will return success (zero) to the calling environment. If you
enter non-conformable units, then units will print a message giving the reduced form of each unit and it
will return failure (nonzero) to the calling environment.
If the ‘--conformable’ option is given, only one unit expression is allowed, and units will print all
units conformable with that expression; it is equivalent to giving ? at the ‘You want:’ prompt. For
example,
units --conformable gauss
B_FIELD tesla
Gs gauss
T tesla
gauss abvolt sec / cm^2
stT stattesla
statT stattesla
stattesla statWb/cm^2
tesla Wb/m^2
If you give more than one unit expression with the ‘--conformable’ option, the program will exit with an
error message and return failure. This option has no effect in interactive mode.
If the ‘--terse’ (‘-t’) option is given with the ‘--conformable’ option, conformable units are shown
without definitions; with the previous example, this would give
units --terse --conformable gauss
B_FIELD
Gs
T
gauss
stT
statT
stattesla
tesla
When the ‘--conformable’ option is not given and you invoke units with only one argument, units will
print the definition of the specified unit. It will return failure if the unit is not defined and
success if the unit is defined.
UNIT DEFINITIONS
The conversion information is read from several units data files: ‘definitions.units’, ‘elements.units’,
‘currency.units’, and ‘cpi.units’, which are usually located in the ‘/usr/share/units’ directory. If you
invoke units with the ‘-V’ option, it will print the location of these files. The default main file
includes definitions for all familiar units, abbreviations and metric prefixes. It also includes many
obscure or archaic units. Many common spelled-out numbers (e.g., ‘seventeen’) are recognized.
Physical Constants
Many constants of nature are defined, including these:
pi ratio of circumference of a circle to its diameter
c speed of light
e charge on an electron
force acceleration of gravity
mole Avogadro's number
water pressure per unit height of water
Hg pressure per unit height of mercury
au astronomical unit
k Boltzman's constant
mu0 permeability of vacuum
epsilon0 permittivity of vacuum
G Gravitational constant
mach speed of sound
The standard data file includes numerous other constants. Also included are the densities of various
ingredients used in baking so that ‘2 cups flour_sifted’ can be converted to ‘grams’. This is not an
exhaustive list. Consult the units data file to see the complete list, or to see the definitions that
are used.
Atomic Masses of the Elements
The data file ‘elements.units’ includes atomic masses for most elements and most known isotopes. If the
mole fractions of constituent isotopes are known, an elemental mass is calculated from the sum of the
products of the mole fractions and the masses of the constituent isotopes. If the mole fractions are not
known, the mass of the most stable isotope—if known—is given as the elemental mass. For radioactive
elements with atomic numbers 95 or greater, the mass number of the most stable isotope is not specified,
because the list of studied isotopes is still incomplete. If no stable isotope is known, no elemental
mass is given, and you will need to choose the most appropriate isotope.
The data are obtained from the US National Institute for Standards and Technology (NIST):
https://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl?ele=&all=all&ascii=ascii2&isotype=all. The
‘elements.units’ file can be generated from these data using the elemcvt command included with the
distribution.
Currency Exchange Rates and Consumer Price Index
The data file ‘currency.units’ includes currency conversion rates; the file ‘cpi.units’ includes the US
Consumer Price Index (CPI), published by the US Bureau of Labor Statistics. The data are updated monthly
by the BLS; see Updating Currency Exchange Rates and CPI for information on updating ‘currency.units’ and
‘cpi.units’.
English Customary Units
English customary units differ in various ways among different regions. In Britain a complex system of
volume measurements featured different gallons for different materials such as a wine gallon and ale
gallon that different by twenty percent. This complexity was swept away in 1824 by a reform that created
an entirely new gallon, the British Imperial gallon defined as the volume occupied by ten pounds of
water. Meanwhile in the USA the gallon is derived from the 1707 Winchester wine gallon, which is 231
cubic inches. These gallons differ by about twenty percent. By default if units runs in the ‘en_GB’
locale you will get the British volume measures. If it runs in the ‘en_US’ locale you will get the US
volume measures. In other locales the default values are the US definitions. If you wish to force
different definitions, then set the environment variable UNITS_ENGLISH to either ‘US’ or ‘GB’ to set the
desired definitions independent of the locale.
Before 1959, the value of a yard (and other units of measure defined in terms of it) differed slightly
among English-speaking countries. In 1959, Australia, Canada, New Zealand, the United Kingdom, the
United States, and South Africa adopted the Canadian value of 1 yard = 0.9144 m (exactly), which was
approximately halfway between the values used by the UK and the US; it had the additional advantage of
making 1 inch = 2.54 cm (exactly). This new standard was termed the International Yard. Australia,
Canada, and the UK then defined all customary lengths in terms of the International Yard (Australia did
not define the furlong or rod); because many US land surveys were in terms of the pre-1959 units, the US
continued to define customary surveyors' units (furlong, chain, rod, pole, perch, and link) in terms of
the previous value for the foot, which was termed the US survey foot. The US defined a US survey mile as
5280 US survey feet, and defined a statute mile as a US survey mile. The US values for these units
differed from the international values by about 2 ppm.
The 1959 redefinition of the foot was legally binding in the US but allowed continued use of the previous
definition of the foot for geodetic surveying. It was assumed that this use would be temporary, but use
persisted, leading to confusion and errors, and it was at odds with the intent of uniform standards.
Since January 1, 2023, the US survey foot has been officially deprecated (85 FR 62698), with its use
limited to historical and legacy applications.
The units program has always used the international values for these units; the legacy US values can be
obtained by using either the ‘US’ or the ‘survey’ prefix. In either case, the simple familiar
relationships among the units are maintained, e.g., 1 ‘furlong’ = 660 ‘ft’, and 1 ‘USfurlong’ = 660
‘USft’, though the metric equivalents differ slightly between the two cases. The ‘US’ prefix or the
‘survey’ prefix can also be used to obtain the US survey mile and the value of the US yard prior to 1959,
e.g., ‘USmile’ or ‘surveymile’ (but not ‘USsurveymile’). To get the US value of the statute mile, use
either ‘USstatutemile’ or ‘USmile’. The pre-1959 UK values for these units can be obtained with the
prefix ‘UK’.
Except for distances that extend over hundreds of miles (such as in the US State Plane Coordinate
System), the differences in the miles are usually insignificant:
You have: 100 surveymile - 100 mile
You want: inch
* 12.672025
/ 0.078913984
The US acre was officially defined in terms of the US survey foot, but units has used a definition based
on the international foot; the units definition is now the same as the official US value. If you want
the previous US acre, use ‘USacre’ and similarly use ‘USacrefoot’ for the previous US version of that
unit. The difference between these units is about 4 parts per million.
Miscellaneous Notes on Unit Definitions
The ‘pound’ is a unit of mass. To get force, multiply by the force conversion unit ‘force’ or use the
shorthand ‘lbf’. (Note that ‘g’ is already taken as the standard abbreviation for the gram.) The unit
‘ounce’ is also a unit of mass. The fluid ounce is ‘fluidounce’ or ‘floz’. When British capacity units
differ from their US counterparts, such as the British Imperial gallon, the unit is defined both ways
with ‘br’ and ‘us’ prefixes. Your locale settings will determine the value of the unprefixed unit.
Currency is prefixed with its country name: ‘belgiumfranc’, ‘britainpound’.
When searching for a unit, if the specified string does not appear exactly as a unit name, then the units
program will try to remove a trailing ‘s’, ‘es’. Next units will replace a trailing ‘ies’ with ‘y’. If
that fails, units will check for a prefix. The database includes all of the standard metric prefixes.
Only one prefix is permitted per unit, so ‘micromicrofarad’ will fail. However, prefixes can appear
alone with no unit following them, so ‘micro*microfarad’ will work, as will ‘micro microfarad’.
To find out which units and prefixes are available, read the default units data files; the main data file
is extensively annotated.
UNIT EXPRESSIONS
Operators
You can enter more complicated units by combining units with operations such as multiplication, division,
powers, addition, subtraction, and parentheses for grouping. You can use the customary symbols for these
operators when units is invoked with its default options. Additionally, units supports some extensions,
including high priority multiplication using a space, and a high priority numerical division operator
(‘|’) that can simplify some expressions.
You multiply units using a space or an asterisk (‘*’). The next example shows both forms:
You have: arabicfoot * arabictradepound * force
You want: ft lbf
* 0.7296
/ 1.370614
You can divide units using the slash (‘/’) or with ‘per’:
You have: furlongs per fortnight
You want: m/s
* 0.00016630986
/ 6012.8727
You can use parentheses for grouping:
You have: (1/2) kg / (kg/meter)
You want: league
* 0.00010356166
/ 9656.0833
White space surrounding operators is optional, so the previous example could have used
‘(1/2)kg/(kg/meter)’. As a consequence, however, hyphenated spelled-out numbers (e.g., ‘forty-two’)
cannot be used; ‘forty-two’ is interpreted as ‘40 - 2’.
Multiplication using a space has a higher precedence than division using a slash and is evaluated left to
right; in effect, the first ‘/’ character marks the beginning of the denominator of a unit expression.
This makes it simple to enter a quotient with several terms in the denominator: ‘J / mol K’. The ‘*’ and
‘/’ operators have the same precedence, and are evaluated left to right; if you multiply with ‘*’, you
must group the terms in the denominator with parentheses: ‘J / (mol * K)’.
The higher precedence of the space operator may not always be advantageous. For example, ‘m/s s/day’ is
equivalent to ‘m / s s day’ and has dimensions of length per time cubed. Similarly, ‘1/2 meter’ refers
to a unit of reciprocal length equivalent to 0.5/meter, perhaps not what you would intend if you entered
that expression. The get a half meter you would need to use parentheses: ‘(1/2) meter’. The ‘*’
operator is convenient for multiplying a sequence of quotients. For example, ‘m/s * s/day’ is equivalent
to ‘m/day’. Similarly, you could write ‘1/2 * meter’ to get half a meter.
The units program supports another option for numerical fractions: you can indicate division of numbers
with the vertical bar (‘|’), so if you wanted half a meter you could write ‘1|2 meter’. You cannot use
the vertical bar to indicate division of non-numerical units (e.g., ‘m|s’ results in an error message).
Powers of units can be specified using the ‘^’ character, as shown in the following example, or by simple
concatenation of a unit and its exponent: ‘cm3’ is equivalent to ‘cm^3’; if the exponent is more than one
digit, the ‘^’ is required. You can also use ‘**’ as an exponent operator.
You have: cm^3
You want: gallons
* 0.00026417205
/ 3785.4118
Concatenation only works with a single unit name: if you write ‘(m/s)2’, units will treat it as
multiplication by 2. When a unit includes a prefix, exponent operators apply to the combination, so
‘centimeter3’ gives cubic centimeters. If you separate the prefix from the unit with any multiplication
operator (e.g., ‘centi meter^3’), the prefix is treated as a separate unit, so the exponent applies only
to the unit without the prefix. The second example is equivalent to ‘centi * (meter^3)’, and gives a
hundredth of a cubic meter, not a cubic centimeter. The units program is limited internally to products
of 99 units; accordingly, expressions like ‘meter^100’ or ‘joule^34’ (represented internally as
‘kg^34 m^68 / s^68’) will fail.
The ‘|’ operator has the highest precedence, so you can write the square root of two thirds as ‘2|3^1|2’.
The ‘^’ operator has the second highest precedence, and is evaluated right to left, as usual:
You have: 5 * 2^3^2
You want:
Definition: 2560
With a dimensionless base unit, any dimensionless exponent is meaningful (e.g., ‘pi^exp(2.371)’). Even
though angle is sometimes treated as dimensionless, exponents cannot have dimensions of angle:
You have: 2^radian
^
Exponent not dimensionless
If the base unit is not dimensionless, the exponent must be a rational number p/q, and the dimension of
the unit must be a power of q, so ‘gallon^2|3’ works but ‘acre^2|3’ fails. An exponent using the slash
(‘/’) operator (e.g., ‘gallon^(2/3)’) is also acceptable; the parentheses are needed because the
precedence of ‘^’ is higher than that of ‘/’. Since units cannot represent dimensions with exponents
greater than 99, a fully reduced exponent must have q < 100. When raising a non-dimensionless unit to a
power, units attempts to convert a decimal exponent to a rational number with q < 100. If this is not
possible units displays an error message:
You have: ft^1.234
Base unit not dimensionless; rational exponent required
A decimal exponent must match its rational representation to machine precision, so ‘acre^1.5’ works but
‘gallon^0.666’ does not.
Sums and Differences of Units
You may sometimes want to add values of different units that are outside the SI. You may also wish to
use units as a calculator that keeps track of units. Sums of conformable units are written with the ‘+’
character, and differences with the ‘-’ character.
You have: 2 hours + 23 minutes + 32 seconds
You want: seconds
* 8612
/ 0.00011611705
You have: 12 ft + 3 in
You want: cm
* 373.38
/ 0.0026782366
You have: 2 btu + 450 ft lbf
You want: btu
* 2.5782804
/ 0.38785542
The expressions that are added or subtracted must reduce to identical expressions in primitive units, or
an error message will be displayed:
You have: 12 printerspoint - 4 heredium
^
Invalid sum of non-conformable units
If you add two values of vastly different scale you may exceed the available precision of floating point
(about 15 digits). The effect is that the addition of the smaller value makes no change to the larger
value; in other words, the smaller value is treated as if it were zero.
You have: lightyear + cm
No warning is given, however. As usual, the precedence for ‘+’ and ‘-’ is lower than that of the other
operators. A fractional quantity such as 2 1/2 cups can be given as ‘(2+1|2) cups’; the parentheses are
necessary because multiplication has higher precedence than addition. If you omit the parentheses, units
attempts to add ‘2’ and ‘1|2 cups’, and you get an error message:
You have: 2+1|2 cups
^
Invalid sum or difference of non-conformable units
The expression could also be correctly written as ‘(2+1/2) cups’. If you write ‘2 1|2 cups’ the space is
interpreted as multiplication so the result is the same as ‘1 cup’.
The ‘+’ and ‘-’ characters sometimes appears in exponents like ‘3.43e+8’. This leads to an ambiguity in
an expression like ‘3e+2 yC’. The unit ‘e’ is a small unit of charge, so this can be regarded as
equivalent to ‘(3e+2) yC’ or ‘(3 e)+(2 yC)’. This ambiguity is resolved by always interpreting ‘+’ and
‘-’ as part of an exponent if possible.
Numbers as Units
For units, numbers are just another kind of unit. They can appear as many times as you like and in any
order in a unit expression. For example, to find the volume of a box that is 2 ft by 3 ft by 12 ft in
steres, you could do the following:
You have: 2 ft 3 ft 12 ft
You want: stere
* 2.038813
/ 0.49048148
You have: $ 5 / yard
You want: cents / inch
* 13.888889
/ 0.072
And the second example shows how the dollar sign in the units conversion can precede the five. Be
careful: units will interpret ‘$5’ with no space as equivalent to ‘dollar^5’.
Built-in Functions
Several built-in functions are provided: ‘sin’, ‘cos’, ‘tan’, ‘asin’, ‘acos’, ‘atan’, ‘sinh’, ‘cosh’,
‘tanh’, ‘asinh’, ‘acosh’, ‘atanh’, ‘exp’, ‘ln’, ‘log’, ‘abs’, ‘round’, ‘floor’, ‘ceil’, ‘factorial’,
‘Gamma’, ‘lnGamma’, ‘erf’, and ‘erfc’; the function ‘lnGamma’ is the natural logarithm of the ‘Gamma’
function.
The ‘sin’, ‘cos’, and ‘tan’ functions require either a dimensionless argument or an argument with
dimensions of angle.
You have: sin(30 degrees)
You want:
Definition: 0.5
You have: sin(pi/2)
You want:
Definition: 1
You have: sin(3 kg)
^
Unit not dimensionless
The other functions on the list require dimensionless arguments. The inverse trigonometric functions
return arguments with dimensions of angle.
The ‘ln’ and ‘log’ functions give natural log and log base 10 respectively. To obtain logs for any
integer base, enter the desired base immediately after ‘log’. For example, to get log base 2 you would
write ‘log2’ and to get log base 47 you could write ‘log47’.
You have: log2(32)
You want:
Definition: 5
You have: log3(32)
You want:
Definition: 3.1546488
You have: log4(32)
You want:
Definition: 2.5
You have: log32(32)
You want:
Definition: 1
You have: log(32)
You want:
Definition: 1.50515
You have: log10(32)
You want:
Definition: 1.50515
If you wish to take roots of units, you may use the ‘sqrt’ or ‘cuberoot’ functions. These functions
require that the argument have the appropriate root. You can obtain higher roots by using fractional
exponents:
You have: sqrt(acre)
You want: feet
* 208.71074
/ 0.0047913202
You have: (400 W/m^2 / stefanboltzmann)^(1/4)
You have:
Definition: 289.80882 K
You have: cuberoot(hectare)
^
Unit not a root
Previous Result
You can insert the result of the previous conversion using the underscore (‘_’). It is useful when you
want to convert the same input to several different units, for example
You have: 2.3 tonrefrigeration
You want: btu/hr
* 27600
/ 3.6231884e-005
You have: _
You want: kW
* 8.0887615
/ 0.12362832
Suppose you want to do some deep frying that requires an oil depth of 2 inches. You have 1/2 gallon of
oil, and want to know the largest-diameter pan that will maintain the required depth. The nonlinear unit
‘circlearea’ gives the radius of the circle (see Other Nonlinear Units, for a more detailed description)
in SI units; you want the diameter in inches:
You have: 1|2 gallon / 2 in
You want: circlearea
0.10890173 m
You have: 2 _
You want: in
* 8.5749393
/ 0.1166189
In most cases, surrounding white space is optional, so the previous example could have used ‘2_’. If ‘_’
follows a non-numerical unit symbol, however, the space is required:
You have: m_
^
Parse error
You can use the ‘_’ symbol any number of times; for example,
You have: m
You want:
Definition: 1 m
You have: _ _
You want:
Definition: 1 m^2
Using ‘_’ before a conversion has been performed (e.g., immediately after invocation) generates an error:
You have: _
^
No previous result; '_' not set
Accordingly, ‘_’ serves no purpose when units is invoked non-interactively.
If units is invoked with the ‘--verbose’ option (see Invoking Units), the value of ‘_’ is not expanded:
You have: mile
You want: ft
mile = 5280 ft
mile = (1 / 0.00018939394) ft
You have: _
You want: m
_ = 1609.344 m
_ = (1 / 0.00062137119) m
You can give ‘_’ at the ‘You want:’ prompt, but it usually is not very useful.
Complicated Unit Expressions
The units program is especially helpful in ensuring accuracy and dimensional consistency when converting
lengthy unit expressions. For example, one form of the Darcy-Weisbach fluid-flow equation is
Delta P = (8 / pi)^2 (rho fLQ^2) / d^5,
where Delta P is the pressure drop, rho is the mass density, f is the (dimensionless) friction factor, L
is the length of the pipe, Q is the volumetric flow rate, and d is the pipe diameter. You might want to
have the equation in the form
Delta P = A1 rho fLQ^2 / d^5
that accepted the user’s normal units; for typical units used in the US, the required conversion could be
something like
You have: (8/pi^2)(lbm/ft^3)ft(ft^3/s)^2(1/in^5)
You want: psi
* 43.533969
/ 0.022970568
The parentheses allow individual terms in the expression to be entered naturally, as they might be read
from the formula. Alternatively, the multiplication could be done with the ‘*’ rather than a space; then
parentheses are needed only around ‘ft^3/s’ because of its exponent:
You have: 8/pi^2 * lbm/ft^3 * ft * (ft^3/s)^2 /in^5
You want: psi
* 43.533969
/ 0.022970568
Without parentheses, and using spaces for multiplication, the previous conversion would need to be
entered as
You have: 8 lb ft ft^3 ft^3 / pi^2 ft^3 s^2 in^5
You want: psi
* 43.533969
/ 0.022970568
Variables Assigned at Run Time
Unit definitions are fixed once units has finished reading the units data file(s), but at run time you
can assign unit expressions to variables whose names begin with an underscore, using the syntax
_name = <unit expression>
This can help manage a long calculation by saving intermediate quantities as variables that you can use
later. For example, to determine the shot-noise-limited signal-to-noise ratio (SNR) of an imaging system
using a helium–neon laser, you could do
You have: _lambda = 632.8 nm # laser wavelength
You have: _nu = c / _lambda # optical frequency
You have: _photon_energy = h * _nu
You have: _power = 550 uW
You have: _photon_count = _power * 500 ns / _photon_energy
You have: _snr = sqrt(_photon_count)
You have: _snr
You want:
Definition: sqrt(_photon_count) = 29597.922
Except for beginning with an underscore, runtime variables follow the same naming rules as units.
Because names beginning with ‘_’ are reserved for these variables and unit names cannot begin with ‘_’,
runtime variables can never hide unit definitions. Runtime variables are undefined until you make an
assignment to them, so if you give a name beginning with an underscore and no assignment has been made,
you get an error message.
When you assign a unit expression to a runtime variable, units checks the expression to determine whether
it is valid, but the resulting definition is stored as a text string, and is not reduced to primitive
units. The text will be processed anew each time you use the variable in a conversion or calculation.
This means that if your definition depends on other runtime variables (or the special variable ‘_’), the
result of calculating with your variable will change if any of those variables change. A dependence need
not be direct.
Continuing the example of the laser above, suppose you have done the calculation as shown. You now
wonder what happens if you switch to an argon laser:
You have: _lambda = 454.6 nm
You have: _snr
You want:
Definition: sqrt(_photon_count) = 25086.651
If you then change the power:
You have: _power = 1 mW
You have: _snr
You want:
Definition: sqrt(_photon_count) = 33826.834
Instead of having to reenter or edit a lengthy expression when you perform another calculation, you need
only enter values that change; in this respect, runtime variables are similar to a spreadsheet.
The more times a variable appears in an expression that depends on it, the greater the benefit of having
a calculation using that expression reflect changes to that variable. For example, the length of a
sidereal day at a given latitude and declination of the Sun is given by
L = acos((sin h - sin ϕ sin δ) /
(cos ϕ cos δ))
where L is the day length, h is the altitude, ϕ is the latitude, and δ is the Sun’s declination.
The length of a solar day is obtained from a sidereal day by multiplying by
siderealday / day
By convention, the Sun’s altitude at rise or set is -50′ to allow for atmospheric refraction and the
semidiameter of its disk. At the summer solstice, the Sun’s declination is approximately 23.44°; to find
the length of the longest day of the year for a latitude of 55°, you could do
You have: _alt = -50 arcmin
You have: _lat = 55 deg
You have: _decl = 23.44 deg
You have: _num = sin(_alt) - sin(_lat) sin(_decl)
You have: _denom = cos(_lat) cos(_decl)
You have: _sday = 2 (acos(_num / _denom) / circle) 24 hr
You have: _day = _sday siderealday / day
You have: _day
You want: hms
17 hr + 19 min + 34.895151 sec
At the winter solstice, the Sun’s declination is approximately -23.44°, so you could calculate the length
of the shortest day of the year using:
You have: _decl = -23.44 deg
You have: _day
You want: hms
7 hr + 8 min + 40.981084 sec
Latitude and declination each appear twice in the expression for _day; the result in the examples above
is updated by changing only the value of the declination.
It’s important to remember that evaluation of runtime variables is delayed, so you cannot make an
assignment that is self-referential. For example, the following does not work:
You have: _decl = 23.44 deg
You have: _decl = -_decl
You have: _decl
Circular unit definition
A runtime variable must be assigned before it can be used in an assignment; in the first of the three
examples above, giving the general equation before the values for _alt, _lat, and _decl had been assigned
would result in an error message.
Backwards Compatibility: ‘*’ and ‘-’
The original units assigned multiplication a higher precedence than division using the slash. This
differs from the usual precedence rules, which give multiplication and division equal precedence, and can
be confusing for people who think of units as a calculator.
The star operator (‘*’) included in this units program has, by default, the same precedence as division,
and hence follows the usual precedence rules. For backwards compatibility you can invoke units with the
‘--oldstar’ option. Then ‘*’ has a higher precedence than division, and the same precedence as
multiplication using the space.
Historically, the hyphen (‘-’) has been used in technical publications to indicate products of units, and
the original units program treated it as a multiplication operator. Because units provides several other
ways to obtain unit products, and because ‘-’ is a subtraction operator in general algebraic expressions,
units treats the binary ‘-’ as a subtraction operator by default. For backwards compatibility use the
‘--product’ option, which causes units to treat the binary ‘-’ operator as a product operator. When ‘-’
is a multiplication operator it has the same precedence as multiplication with a space, giving it a
higher precedence than division.
When ‘-’ is used as a unary operator it negates its operand. Regardless of the units options, if ‘-’
appears after ‘(’ or after ‘+’, then it will act as a negation operator. So you can always compute 20
degrees minus 12 minutes by entering ‘20 degrees + -12 arcmin’. You must use this construction when you
define new units because you cannot know what options will be in force when your definition is processed.
NONLINEAR UNIT CONVERSIONS
Nonlinear units are represented using functional notation. They make possible nonlinear unit conversions
such as temperature.
Temperature Conversions
Conversions between temperatures are different from linear conversions between temperature increments—see
the example below. The absolute temperature conversions are handled by units starting with ‘temp’, and
you must use functional notation. The temperature-increment conversions are done using units starting
with ‘deg’ and they do not require functional notation.
You have: tempF(45)
You want: tempC
7.2222222
You have: 45 degF
You want: degC
* 25
/ 0.04
Think of ‘tempF(x)’ not as a function but as a notation that indicates that x should have units of
‘tempF’ attached to it. See Defining Nonlinear Units. The first conversion shows that if it’s 45
degrees Fahrenheit outside, it’s 7.2 degrees Celsius. The second conversion indicates that a change of
45 degrees Fahrenheit corresponds to a change of 25 degrees Celsius. The conversion from ‘tempF(x)’ is
to absolute temperature, so that
You have: tempF(45)
You want: degR
* 504.67
/ 0.0019814929
gives the same result as
You have: tempF(45)
You want: tempR
* 504.67
/ 0.0019814929
But if you convert ‘tempF(x)’ to ‘degC’, the output is probably not what you expect:
You have: tempF(45)
You want: degC
* 280.37222
/ 0.0035666871
The result is the temperature in K, because ‘degC’ is defined as ‘K’, the kelvin. For consistent results,
use the ‘tempX’ units when converting to a temperature rather than converting a temperature increment.
The ‘tempC()’ and ‘tempF()’ definitions are limited to positive absolute temperatures, and giving a value
that would result in a negative absolute temperature generates an error message:
You have: tempC(-275)
^
Argument of function outside domain
US Consumer Price Index
units includes the US Consumer Price Index published by the US Bureau of Labor Statistics. Several
functions that use this value are provided: ‘cpi’, ‘cpi_now’, ‘inflation_since’, and ‘dollars_in’.
The ‘cpi’ function gives the CPI for a specified decimal year. A decimal year is given as the year plus
the fractional part of the year; because of leap years and the different lengths of months, calculating
an exact value for the fractional part can be tedious, but for the purposes of CPI, an approximate value
is usually adequate. For example, 1 January 2000 is 2000.0, 1 April 2000 is 2000.25, 1 July 2000 is
2000.4986, and 1 October 2000 is 2000.75. Note also that the CPI data update monthly; values in between
months are linearly interpolated.
In the middle of 1975, the CPI was
You have: cpi(1975.5)
You want:
Definition: 53.6
The value of the CPI for the previous month is usually published toward the the month; the latest value
of the CPI is available with ‘cpi_now’. On 7 January 2024, the value was
You have: cpi_now
You want:
Definition: UScpi_now = 307.051
This means that the CPI was 307.015 on 1 December 2023. The ‘cpi_now’ variable can only present the most
recent data available, so it can lag the current CPI by several weeks. The decimal year of the last
update is available with ‘cpi_lastdate’.
The ‘inflation_since’ function provides a convenient way to determine the inflation factor from a
specified decimal year to the latest value in the CPI table. For example, on 7 January 2024:
You have: inflation_since(1970)
You want:
Definition: 8.1445889
In other words, goods that cost 1 US$ in 1970 would cost 8.14 US$ on 1 December 2023.
The ‘inflation_since’ function can be used to determine an annual rate of inflation. The earliest US CPI
data are from about 1913.1; the approximate time between then and 7 January 2024 is 110.9 years. The
approximate annual inflation rate for that period is then
You have: inflation_since(1913.1)^1|110.9 - 1
You want: %
* 3.1548115
/ 0.31697614
The inflation rate for any time period can be found from the ratio of the CPI at the end of the period to
that of the beginning:
You have: (cpi(1982)/cpi(1972))^1|10 - 1
You want: %
* 8.6247033
/ 0.11594602
The period 1972–1982 was indeed one of high inflation.
The ‘dollars_in’ function is similar to ‘inflation_since’ but its output is in US$ rather than
dimensionless:
You have: dollars_in(1970)
You want:
Definition: 8.1445889 US$
A typical use might be
You have: 250 dollars_in(1970)
You want: $
* 2036.1472
/ 0.00049112362
Because ‘dollars_in’ includes the units, you should not include them at the ‘You have:’ prompt. You can
also use ‘dollars_in’ to convert between two specified years:
You have: 250 dollars_in(1970)
You want: dollars_in(1950)
* 156.49867
/ 0.0063898305
which shows that 250 US$ in 1970 would have equivalent purchasing power to 156 US$ in 1950.
Other Nonlinear Units
Some other examples of nonlinear units are numerous different ring sizes and wire gauges, the grit sizes
used for abrasives, the decibel scale, shoe size, scales for the density of sugar (e.g., baume). The
standard data file also supplies units for computing the area of a circle and the volume of a sphere.
See the standard units data file for more details. Wire gauges with multiple zeroes are signified using
negative numbers where two zeroes is ‘-1’. Alternatively, you can use the synonyms ‘g00’, ‘g000’, and so
on that are defined in the standard units data file.
You have: wiregauge(11)
You want: inches
* 0.090742002
/ 11.020255
You have: brwiregauge(g00)
You want: inches
* 0.348
/ 2.8735632
You have: 1 mm
You want: wiregauge
18.201919
You have: grit_P(600)
You want: grit_ansicoated
342.76923
The last example shows the conversion from P graded sand paper, which is the European standard and may be
marked “P600” on the back, to the USA standard.
You can compute the area of a circle using the nonlinear unit, ‘circlearea’. You can also do this using
the circularinch or circleinch. The next example shows two ways to compute the area of a circle with a
five inch radius and one way to compute the volume of a sphere with a radius of one meter.
You have: circlearea(5 in)
You want: in2
* 78.539816
/ 0.012732395
You have: 10^2 circleinch
You want: in2
* 78.539816
/ 0.012732395
You have: spherevol(meter)
You want: ft3
* 147.92573
/ 0.0067601492
The inverse of a nonlinear conversion is indicated by prefixing a tilde (‘~’) to the nonlinear unit name:
You have: ~wiregauge(0.090742002 inches)
You want:
Definition: 11
You can give a nonlinear unit definition without an argument or parentheses, and press Enter at the
‘You want:’ prompt to get the definition of a nonlinear unit; if the definition is not valid for all real
numbers, the range of validity is also given. If the definition requires specific units this information
is also displayed:
You have: tempC
Definition: tempC(x) = x K + stdtemp
defined for x >= -273.15
You have: ~tempC
Definition: ~tempC(tempC) = (tempC +(-stdtemp))/K
defined for tempC >= 0 K
You have: circlearea
Definition: circlearea(r) = pi r^2
r has units m
To see the definition of the inverse use the ‘~’ notation. In this case the parameter in the functional
definition will usually be the name of the unit. Note that the inverse for ‘tempC’ shows that it
requires units of ‘K’ in the specification of the allowed range of values. Nonlinear unit conversions
are described in more detail in Defining Nonlinear Units.
UNIT LISTS: CONVERSION TO SUMS OF UNITS
Outside of the SI, it is sometimes desirable to convert a single unit to a sum of units—for example, feet
to feet plus inches. The conversion from sums of units was described in Sums and Differences of Units,
and is a simple matter of adding the units with the ‘+’ sign:
You have: 12 ft + 3 in + 3|8 in
You want: ft
* 12.28125
/ 0.081424936
Although you can similarly write a sum of units to convert to, the result will not be the conversion to
the units in the sum, but rather the conversion to the particular sum that you have entered:
You have: 12.28125 ft
You want: ft + in + 1|8 in
* 11.228571
/ 0.089058524
The unit expression given at the ‘You want:’ prompt is equivalent to asking for conversion to multiples
of ‘1 ft + 1 in + 1|8 in’, which is 1.09375 ft, so the conversion in the previous example is equivalent
to
You have: 12.28125 ft
You want: 1.09375 ft
* 11.228571
/ 0.089058524
In converting to a sum of units like miles, feet and inches, you typically want the largest integral
value for the first unit, followed by the largest integral value for the next, and the remainder
converted to the last unit. You can do this conversion easily with units using a special syntax for
lists of units. You must list the desired units in order from largest to smallest, separated by the
semicolon (‘;’) character:
You have: 12.28125 ft
You want: ft;in;1|8 in
12 ft + 3 in + 3|8 in
The conversion always gives integer coefficients on the units in the list, except possibly the last unit
when the conversion is not exact:
You have: 12.28126 ft
You want: ft;in;1|8 in
12 ft + 3 in + 3.00096 * 1|8 in
The order in which you list the units is important:
You have: 3 kg
You want: oz;lb
105 oz + 0.051367866 lb
You have: 3 kg
You want: lb;oz
6 lb + 9.8218858 oz
Listing ounces before pounds produces a technically correct result, but not a very useful one. You must
list the units in descending order of size in order to get the most useful result.
Ending a unit list with the separator ‘;’ has the same effect as repeating the last unit on the list, so
‘ft;in;1|8 in;’ is equivalent to ‘ft;in;1|8 in;1|8 in’. With the example above, this gives
You have: 12.28126 ft
You want: ft;in;1|8 in;
12 ft + 3 in + 3|8 in + 0.00096 * 1|8 in
in effect separating the integer and fractional parts of the coefficient for the last unit. If you
instead prefer to round the last coefficient to an integer you can do this with the ‘--round’ (‘-r’)
option. With the previous example, the result is
You have: 12.28126 ft
You want: ft;in;1|8 in
12 ft + 3 in + 3|8 in (rounded down to nearest 1|8 in)
When you use the ‘-r’ option, repeating the last unit on the list has no effect (e.g., ‘ft;in;1|8 in;1|8
in’ is equivalent to ‘ft;in;1|8 in’), and hence neither does ending a list with a ‘;’. With a single
unit and the ‘-r’ option, a terminal ‘;’ does have an effect: it causes units to treat the single unit as
a list and produce a rounded value for the single unit. Without the extra ‘;’, the ‘-r’ option has no
effect on single unit conversions. This example shows the output using the ‘-r’ option:
You have: 12.28126 ft
You want: in
* 147.37512
/ 0.0067854058
You have: 12.28126 ft
You want: in;
147 in (rounded down to nearest in)
Each unit that appears in the list must be conformable with the first unit on the list, and of course the
listed units must also be conformable with the unit that you enter at the ‘You have:’ prompt.
You have: meter
You want: ft;kg
^
conformability error
ft = 0.3048 m
kg = 1 kg
You have: meter
You want: lb;oz
conformability error
1 m
0.45359237 kg
In the first case, units reports the disagreement between units appearing on the list. In the second
case, units reports disagreement between the unit you entered and the desired conversion. This
conformability error is based on the first unit on the unit list.
Other common candidates for conversion to sums of units are angles and time:
You have: 23.437754 deg
You want: deg;arcmin;arcsec
23 deg + 26 arcmin + 15.9144 arcsec
You have: 7.2319 hr
You want: hr;min;sec
7 hr + 13 min + 54.84 sec
Some applications for unit lists may be less obvious. Suppose that you have a postal scale and wish to
ensure that it’s accurate at 1 oz, but have only metric calibration weights. You might try
You have: 1 oz
You want: 100 g;50 g; 20 g;10 g;5 g;2 g;1 g;
20 g + 5 g + 2 g + 1 g + 0.34952312 * 1 g
You might then place one each of the 20 g, 5 g, 2 g, and 1 g weights on the scale and hope that it
indicates close to
You have: 20 g + 5 g + 2 g + 1 g
You want: oz;
0.98767093 oz
Appending ‘;’ to ‘oz’ forces a one-line display that includes the unit; here the integer part of the
result is zero, so it is not displayed.
If a non-empty list item differs vastly in scale from the quantity from which the list is to be
converted, you may exceed the available precision of floating point (about 15 digits), in which case you
will get a warning, e.g.,
You have: lightyear
You want: mile;100 inch;10 inch;mm;micron
5.8786254e+12 mile + 390 * 100 inch (at 15-digit precision limit)
Cooking Measure
In North America, recipes for cooking typically measure ingredients by volume, and use units that are not
always convenient multiples of each other. Suppose that you have a recipe for 6 and you wish to make a
portion for 1. If the recipe calls for 2 1/2 cups of an ingredient, you might wish to know the
measurements in terms of measuring devices you have available, you could use units and enter
You have: (2+1|2) cup / 6
You want: cup;1|2 cup;1|3 cup;1|4 cup;tbsp;tsp;1|2 tsp;1|4 tsp
1|3 cup + 1 tbsp + 1 tsp
By default, if a unit in a list begins with fraction of the form 1|x and its multiplier is an integer,
the fraction is given as the product of the multiplier and the numerator; for example,
You have: 12.28125 ft
You want: ft;in;1|8 in;
12 ft + 3 in + 3|8 in
In many cases, such as the example above, this is what is wanted, but sometimes it is not. For example,
a cooking recipe for 6 might call for 5 1/4 cup of an ingredient, but you want a portion for 2, and your
1-cup measure is not available; you might try
You have: (5+1|4) cup / 3
You want: 1|2 cup;1|3 cup;1|4 cup
3|2 cup + 1|4 cup
This result might be fine for a baker who has a 1 1/2-cup measure (and recognizes the equivalence), but
it may not be as useful to someone with more limited set of measures, who does want to do additional
calculations, and only wants to know “How many 1/2-cup measures to I need to add?” After all, that’s
what was actually asked. With the ‘--show-factor’ option, the factor will not be combined with a unity
numerator, so that you get
You have: (5+1|4) cup / 3
You want: 1|2 cup;1|3 cup;1|4 cup
3 * 1|2 cup + 1|4 cup
A user-specified fractional unit with a numerator other than 1 is never overridden, however—if a unit
list specifies ‘3|4 cup;1|2 cup’, a result equivalent to 1 1/2 cups will always be shown as ‘2 * 3|4 cup’
whether or not the ‘--show-factor’ option is given.
Unit List Aliases
A unit list such as
cup;1|2 cup;1|3 cup;1|4 cup;tbsp;tsp;1|2 tsp;1|4 tsp
can be tedious to enter. The units program provides shorthand names for some common combinations:
hms time: hours, minutes, seconds
dms angle: degrees, minutes, seconds
time time: years, days, hours, minutes and seconds
usvol US cooking volume: cups and smaller
uswt US weight: pounds and ounces
ftin length: feet, inches and 1/8 inches
inchfine length: inches subdivided to 1/64 inch
Using these shorthands, or unit list aliases, you can do the following conversions:
You have: anomalisticyear
You want: time
1 year + 25 min + 3.4653216 sec
You have: 1|6 cup
You want: usvol
2 tbsp + 2 tsp
You can define your own unit list aliases; see Defining Unit List Aliases.
You cannot combine a unit list alias with other units: it must appear alone at the ‘You want:’ prompt.
You can display the definition of a unit list alias by entering it at the ‘You have:’ prompt:
You have: dms
Definition: unit list, deg;arcmin;arcsec
When you specify compact output with ‘--compact’, ‘--terse’ or ‘-t’ and perform conversion to a unit
list, units lists the conversion factors for each unit in the list, separated by semicolons.
You have: year
You want: day;min;sec
365;348;45.974678
Unlike the case of regular output, zeros are included in this output list:
You have: liter
You want: cup;1|2 cup;1|4 cup;tbsp
4;0;0;3.6280454
ALTERNATIVE UNIT SYSTEMS
CGS Units
The SI—an extension of the MKS (meter–kilogram–second) system—has largely supplanted the older CGS
(centimeter–gram–second) system, but CGS units are still used in a few specialized fields, especially in
physics where they lead to a more elegant formulation of Maxwell’s equations. Conversions between SI and
CGS involving mechanical units are straightforward, involving powers of 10 (e.g., 1 m = 100 cm).
Conversions involving electromagnetic units are more complicated, and units supports four different
systems of CGS units: electrostatic units (ESU), electromagnetic units (EMU), the Gaussian system and the
Heaviside–Lorentz system. The differences between these systems arise from different choices made for
proportionality constants in electromagnetic equations. Coulomb’s law gives electrostatic force between
two charges separated by a distance delim $$ r:
F = k_C q_1 q_2 / r^2.
Ampere’s law gives the electromagnetic force per unit length between two current-carrying conductors
separated by a distance r:
F/l = 2 k_A I_1 I_2 / r.
The two constants, k_C and k_A, are related by the square of the speed of light: k_A = k_C / c^2.
In the SI, the constants have dimensions, and an additional base unit, the ampere, measures electric
current. The CGS systems do not define new base units, but express charge and current as derived units
in terms of mass, length, and time. In the ESU system, the constant for Coulomb’s law is chosen to be
unity and dimensionless, which defines the unit of charge. In the EMU system, the constant for Ampere’s
law is chosen to be unity and dimensionless, which defines a unit of current. The Gaussian system
usually uses the ESU units for charge and current; it chooses another constant so that the units for the
electric and magnetic fields are the same. The Heaviside–Lorentz system is “rationalized” so that
factors of 4{pi} do not appear in Maxwell’s equations. The SI system is similarly rationalized, but the
other CGS systems are not. In the Heaviside–Lorentz (HLU) system the factor of 4{pi} appears in
Coulomb’s law instead; this system differs from the Gaussian system by factors of the square root of
4{pi}
The dimensions of electrical quantities in the various CGS systems are different from the SI dimensions
for the same units; strictly, conversions between these systems and SI are not possible. But units in
different systems relate to the same physical quantities, so there is a correspondence between these
units. The units program defines the units so that you can convert between corresponding units in the
various systems.
The CGS definitions involve cm^(1/2) and g^(1/2), which is problematic because units does not normally
support fractional roots of base units. The ‘--units’ (‘-u’) option allows selection of a CGS unit
system and works around this restriction by introducing base units for the square roots of length and
mass: ‘sqrt_cm’ and ‘sqrt_g’. The centimeter then becomes ‘sqrt_cm^2’ and the gram, ‘sqrt_g^2’. This
allows working from equations using the units in the CGS system, and enforcing dimensional conformity
within that system. Recognized CGS arguments to the ‘--units’ option are ‘gauss[ian]’, ‘esu’, ‘emu’,
‘lhu’; the argument is case insensitive. You can also give ‘si’ which just enforces the default SI mode
and displays ‘(SI)’ at the ‘You have:’ prompt to emphasize the units mode. Some other types of units are
also supported as described below. Giving an unrecognized system generates a warning, and units uses SI
units.
The changes resulting from the ‘--units’ option are actually controlled by the UNITS_SYSTEM environment
variable. If you frequently work with one of the supported CGS units systems, you may set this
environment variable rather than giving the ‘--units’ option at each invocation. As usual, an option
given on the command line overrides the setting of the environment variable. For example, if you would
normally work with Gaussian units but might occasionally work with SI, you could set UNITS_SYSTEM to
‘gaussian’ and specify SI with the ‘--units’ option. Unlike the argument to the ‘--units’ option, the
value of UNITS_SYSTEM is case sensitive, so setting a value of ‘EMU’ will have no effect other than to
give an error message and set SI units.
The CGS definitions appear as conditional settings in the standard units data file, which you can consult
for more information on how these units are defined, or on how to define an alternate units system.
The ESU system derives the electromagnetic units from its unit of charge, the statcoulomb, which is
defined from Coulomb’s law. The statcoulomb equals dyne^(1/2) cm, or cm^(3/2) g^(1/2) s^(−1). The unit
of current, the statampere, is statcoulomb sec, analogous to the relationship in SI. Other electrical
units are then derived in a manner similar to that for SI units; the units use the SI names prefixed by
‘stat-’, e.g., ‘statvolt’ or ‘statV’. The prefix ‘st-’ is also recognized (e.g., ‘stV’).
The EMU system derives the electromagnetic units from its unit of current, the abampere, which is defined
in terms of Ampere’s law. The abampere is equal to dyne^(1/2), or cm^(1/2) g^(1/2) s^(−1). delim off
The unit of charge, the abcoulomb, is abampere sec, again analogous to the SI relationship. Other
electrical units are then derived in a manner similar to that for SI units; the units use the SI names
prefixed by ‘ab-’, e.g., ‘abvolt’ or ‘abV’. The magnetic field units include the gauss, the oersted and
the maxwell.
The Gaussian units system, which was also known as the Symmetric System, uses the same charge and current
units as the ESU system (e.g., ‘statC’, ‘statA’); it differs by defining the magnetic field so that it
has the same units as the electric field. The resulting magnetic field units are the same ones used in
the EMU system: the gauss, the oersted and the maxwell.
The Heaviside–Lorentz system appears to lack named units. We define five basic units, ‘hlu_charge’,
‘hlu_current’, ‘hlu_volt’, ‘hlu_efield’ and ‘hlu_bfield’ for conversions with this system. It is
important to remember that with all of the CGS systems, the units may look the same but mean something
different. The HLU system and Gaussian systems both measure magnetic field using the same CGS
dimensions, but the amount of magnetic field with the same units is different in the two systems.
The CGS systems define units that measure the same thing but may have conflicting dimensions.
Furthermore, the dimensions of the electromagnetic CGS units are never compatible with SI. But if you
measure charge in two different systems you have measured the same physical thing, so there is a
correspondence between the units in the different systems, and units supports conversions between
corresponding units. When running with SI, units defines all of the CGS units in terms of SI. When you
select a CGS system, units defines the SI units and the other CGS system units in terms of the system you
have selected.
(Gaussian) You have: statA
You want: abA
* 3.335641e-11
/ 2.9979246e+10
(Gaussian) You have: abA
You want: sqrt(dyne)
conformability error
2.9979246e+10 sqrt_cm^3 sqrt_g / s^2
1 sqrt_cm sqrt_g / s
In the above example, units converts between the current units statA and abA even though the abA, from
the EMU system, has incompatible dimensions. This works because in Gaussian mode, the abA is defined in
terms of the statA, so it does not have the correct definition for EMU; consequently, you cannot convert
the abA to its EMU definition.
One challenge of conversion is that because the CGS system has fewer base units, quantities that have
different dimensions in SI may have the same dimension in a CGS system. And yet, they may not have the
same conversion factor. For example, the unit for the E field and B fields are the same in the Gaussian
system, but the conversion factors to SI are quite different. This means that correct conversion is only
possible if you keep track of what quantity is being measured. You cannot convert statV/cm to SI without
indicating which type of field the unit measures. To aid in dimensional analysis, units defines various
dimension units such as ‘LENGTH’, ‘TIME’, and ‘CHARGE’ to be the appropriate dimension in SI. The
electromagnetic dimensions such as ‘B_FIELD’ or ‘E_FIELD’ may be useful aids both for conversion and
dimensional analysis in CGS. You can convert them to or from CGS in order to perform SI conversions that
in some cases will not work directly due to dimensional incompatibilities. This example shows how the
Gaussian system uses the same units for all of the fields, but they all have different conversion factors
with SI.
(Gaussian) You have: statV/cm
You want: E_FIELD
* 29979.246
/ 3.335641e-05
(Gaussian) You have: statV/cm
You want: B_FIELD
* 0.0001
/ 10000
(Gaussian) You have: statV/cm
You want: H_FIELD
* 79.577472
/ 0.012566371
(Gaussian) You have: statV/cm
You want: D_FIELD
* 2.6544187e-07
/ 3767303.1
The next example shows that the oersted cannot be converted directly to the SI unit of magnetic field,
A/m, because the dimensions conflict. We cannot redefine the ampere to make this work because then it
would not convert with the statampere. But you can still do this conversion as shown below.
(Gaussian) You have: oersted
You want: A/m
conformability error
1 sqrt_g / s sqrt_cm
29979246 sqrt_cm sqrt_g / s^2
(Gaussian) You have: oersted
You want: H_FIELD
* 79.577472
/ 0.012566371
Natural Units
Like the CGS units, “natural” units are an alternative to the SI system used primarily physicists in
different fields, with different systems tailored to different fields of study. These systems are
“natural” because the base measurements are defined using physical constants instead of arbitrary values
such as the meter or second. In different branches of physics, different physical constants are more
fundamental, which has given rise to a variety of incompatible natural unit systems.
The supported systems are the “natural” units (which seem to have no better name) used in high energy
physics and cosmology, the Planck units, often used by scientists working with gravity, and the Hartree
atomic units are favored by those working in physical chemistry and condensed matter physics.
You can select the various natural units using the ‘--units’ option in the same way that you select the
CGS units. The “natural” units come in two types, a rationalized system derived from the
Heaviside–Lorentz units and an unrationalized system derived from the Gaussian system. You can select
these using ‘natural’ and ‘natural-gauss’ respectively. For conversions in SI mode, several unit names
starting with ‘natural’ are available. This “natural” system is defined by setting {hbar}, c and the
Boltzman constant to 1. Only a single base unit remains: the electron volt.
The Planck units exist in a variety of forms, and units supports two. Both supported forms are
rationalized, in that factors of 4{pi} do not appear in Maxwell’s equations. However, Planck units can
also differ based on how the gravitational constant is treated. This system is similar to the natural
units in that c, {hbar}, and Boltzman’s constant are set to 1, but in this system, Newton’s gravitational
constant, G is also fixed. In the “reduced” Planck system, delim $$ 8{pi}G = 1 whereas in the unreduced
system G = 1. The reduced system eliminates factors of 8{pi} delim off from the Einstein field equations
for gravitation, so this is similar to the process of forming rationalized units to simplify Maxwell’s
equations. To obtain the unreduced system use the name ‘planck’ and for the reduced Planck units,
‘planck-red’. Units such as ‘planckenergy’ and ‘planckenergy_red’ enable you to convert the unreduced
and reduced Planck energy unit in SI mode between the various systems. In Planck units, all measurements
are dimensionless.
The final natural unit system is the Hartree atomic units. Like the Planck units, all measurements in
the Hartree units are dimensionless, but this system is defined by defined from completely different
physical constants: the electron mass, Planck’s constant, the electron charge, and the Coulomb constant
are the defining physical quantities, which are all set to unity. To invoke this system with the
‘--units’ option use the name ‘hartree’.
Prompt Prefix
If a unit system is specified with the ‘--units’ option, the selected system’s name is prepended to the
‘You have:’ prompt as a reminder, e.g.,
(Gaussian) You have: stC
You want:
Definition: statcoulomb = sqrt(dyne) cm = 1 sqrt_cm^3 sqrt_g / s
You can suppressed the prefix by including a line
!prompt
with no argument in a site or personal units data file. The prompt can be conditionally suppressed by
including such a line within ‘!var’ ... ‘!endvar’ constructs, e.g.,
!var UNITS_SYSTEM gaussian gauss
!prompt
!endvar
This might be appropriate if you normally use Gaussian units and find the prefix distracting but want to
be reminded when you have selected a different CGS system.
LOGGING CALCULATIONS
The ‘--log’ option allows you to save the results of calculations in a file; this can be useful if you
need a permanent record of your work. For example, the fluid-flow conversion in Complicated Unit
Expressions, is lengthy, and if you were to use it in designing a piping system, you might want a record
of it for the project file. If the interactive session
# Conversion factor A1 for pressure drop
# dP = A1 rho f L Q^2/d^5
You have: (8/pi^2) (lbm/ft^3)ft(ft^3/s)^2(1/in^5) # Input units
You want: psi
* 43.533969
/ 0.022970568
were logged, the log file would contain
### Log started Fri Oct 02 15:55:35 2015
# Conversion factor A1 for pressure drop
# dP = A1 rho f L Q^2/d^5
From: (8/pi^2) (lbm/ft^3)ft(ft^3/s)^2(1/in^5) # Input units
To: psi
* 43.533969
/ 0.022970568
The time is written to the log file when the file is opened.
The use of comments can help clarify the meaning of calculations for the log. The log includes
conformability errors between the units at the ‘You have:’ and ‘You want:’ prompts, but not other errors,
including lack of conformability of items in sums or differences or among items in a unit list. For
example, a conversion between zenith angle and elevation angle could involve
You have: 90 deg - (5 deg + 22 min + 9 sec)
^
Invalid sum or difference of non-conformable units
You have: 90 deg - (5 deg + 22 arcmin + 9 arcsec)
You want: dms
84 deg + 37 arcmin + 51 arcsec
You have: _
You want: deg
* 84.630833
/ 0.011816024
You have:
The log file would contain
From: 90 deg - (5 deg + 22 arcmin + 9 arcsec)
To: deg;arcmin;arcsec
84 deg + 37 arcmin + 51 arcsec
From: _
To: deg
* 84.630833
/ 0.011816024
The initial entry error (forgetting that minutes have dimension of time, and that arcminutes must be used
for dimensions of angle) does not appear in the output. When converting to a unit list alias, units
expands the alias in the log file.
The ‘From:’ and ‘To:’ tags are written to the log file even if the ‘--quiet’ option is given. If the log
file exists when units is invoked, the new results are appended to the log file. The time is written to
the log file each time the file is opened. The ‘--log’ option is ignored when units is used non-
interactively.
INVOKING UNITS
You invoke units like this:
units [options] [from-unit [to-unit]]
If the from-unit and to-unit are omitted, the program will use interactive prompts to determine which
conversions to perform. See Interactive Use. If both from-unit and to-unit are given, units will print
the result of that single conversion and then exit. If only from-unit appears on the command line, units
will display the definition of that unit and exit. Units specified on the command line may need to be
quoted to protect them from shell interpretation and to group them into two arguments. Note also that
the ‘--quiet’ option is enabled by default if you specify from-unit on the command line. See Command
Line Use.
The default behavior of units can be changed by various options given on the command line. In most
cases, the options may be given in either short form (a single ‘-’ followed by a single character) or
long form (‘--’ followed by a word or hyphen-separated words). Short-form options are cryptic but
require less typing; long-form options require more typing but are more explanatory and may be more
mnemonic. With long-form options you need only enter sufficient characters to uniquely identify the
option to the program. For example, ‘--out %f’ works, but ‘--o %f’ fails because units has other long
options beginning with ‘o’. However, ‘--q’ works because ‘--quiet’ is the only long option beginning
with ‘q’.
Some options require arguments to specify a value (e.g., ‘-d 12’ or ‘--digits 12’). Short-form options
that do not take arguments may be concatenated (e.g., ‘-erS’ is equivalent to ‘-e -r -S’); the last
option in such a list may be one that takes an argument (e.g., ‘-ed 12’). With short-form options, the
space between an option and its argument is optional (e.g., ‘-d12’ is equivalent to ‘-d 12’). Long-form
options may not be concatenated, and the space between a long-form option and its argument is required.
Short-form and long-form options may be intermixed on the command line. Options may be given in any
order, but when incompatible options (e.g., ‘--output-format’ and ‘--exponential’) are given in
combination, behavior is controlled by the last option given. For example, ‘-o%.12f -e’ gives
exponential format with the default eight significant digits).
The following options are available:
-c, --check
Check that all units and prefixes defined in units data files reduce to primitive units. Display
a list of all units that cannot be reduced and a list of units with circular definitions. Also
display some other diagnostics about suspicious definitions in the units data file. Only
definitions active in the current locale are checked. You should always run units with this
option after modifying a units data file.
Some errors may hide other errors, so you should run units with this option again after correcting
any errors, and keep doing so until there are no errors.
--check-verbose, --verbose-check
Like the ‘--check’ option, this option displays a list of units that cannot be reduced. But it
also lists the units as they are checked. Because the ‘--check’ option now catches circular unit
definitions that previously caused units to hang, this option is no longer necessary. It is
retained only for compatibility with previous versions.
-d ndigits, --digits ndigits
Set the number of significant digits in the output to the value specified (which must be greater
than zero). For example, ‘-d 12’ sets the number of significant digits to 12. With exponential
output units displays one digit to the left of the decimal point and eleven digits to the right of
the decimal point. On most systems, the maximum number of internally meaningful digits is 15; if
you specify a greater number than your system’s maximum, units will print a warning and set the
number to the largest meaningful value. To directly set the maximum value, give an argument of
max (e.g., ‘-d max’). Be aware, of course, that “significant” here refers only to the display of
numbers; if results depend on physical constants not known to this precision, the physically
meaningful precision may be less than that shown. The ‘--digits’ option conflicts with the
‘--output-format’ option.
-e, --exponential
Set the numeric output format to exponential (i.e., scientific notation), like that used in the
Unix units program. The default precision is eight significant digits (seven digits to the right
of the decimal point); this can be changed with the ‘--digits’ option. The ‘--exponential’ option
conflicts with the ‘--output-format’ option.
-o format, --output-format format
This option affords complete control over the numeric output format using the specified format.
The format is a single floating point numeric format for the printf function in the C programming
language. All compilers support the format types ‘g’ and ‘G’ to specify significant digits, ‘e’
and ‘E’ for scientific notation, and ‘f’ for fixed-point decimal. The ISO C99 standard introduced
the ‘F’ type for fixed-point decimal and the ‘a’ and ‘A’ types for hexadecimal floating point;
these types are allowed with compilers that support them. The default format is ‘%.8g’; for
greater precision, you could specify ‘-o %.15g’. See Numeric Output Format and the documentation
for printf for more detailed descriptions of the format specification. The ‘--output-format’
option affords the greatest control of the output appearance, but requires at least rudimentary
knowledge of the printf format syntax. If you don’t want to bother with the printf syntax, you
can specify greater precision more simply with the ‘--digits’ option or select exponential format
with ‘--exponential’. The ‘--output-format’ option is incompatible with the ‘--exponential’ and
‘--digits’ options.
-f filename, --file filename
Instruct units to load the units file filename. You can specify up to 25 units files on the
command line. When you use this option, units will load only the files you list on the command
line; it will not load the standard file or your personal units file unless you explicitly list
them. If filename is the empty string (‘-f ""’), the default main units file (or that specified
by UNITSFILE) will be loaded in addition to any others specified with ‘-f’.
-L logfile, --log logfile
Save the results of calculations in the file logfile; this can be useful if it is important to
have a record of unit conversions or other calculations that are to be used extensively or in a
critical activity such as a program or design project. If logfile exits, the new results are
appended to the file. This option is ignored when units is used non-interactively. See Logging
Calculations for a more detailed description and some examples.
-H filename, --history filename
Instruct units to save history to filename, so that a record of your commands is available for
retrieval across different units invocations. To prevent the history from being saved set
filename to the empty string (‘-H ""’). This option has no effect if readline is not available.
-h, --help
Print out a summary of the options for units.
-m, --minus
Causes ‘-’ to be interpreted as a subtraction operator. This is the default behavior.
-p, --product
Causes ‘-’ to be interpreted as a multiplication operator when it has two operands. It will act
as a negation operator when it has only one operand: ‘(-3)’. By default ‘-’ is treated as a
subtraction operator.
--oldstar
Causes ‘*’ to have the old-style precedence, higher than the precedence of division so that
‘1/2*3’ will equal ‘1/6’.
--newstar
Forces ‘*’ to have the new (default) precedence that follows the usual rules of algebra: the
precedence of ‘*’ is the same as the precedence of ‘/’, so that ‘1/2*3’ will equal ‘3/2’.
-r, --round
When converting to a combination of units given by a unit list, round the value of the last unit
in the list to the nearest integer.
-S, --show-factor
When converting to a combination of units specified in a list, always show a non-unity factor
before a unit that begins with a fraction with a unity denominator. By default, if the unit in a
list begins with fraction of the form 1|x and its multiplier is an integer other than 1, the
fraction is given as the product of the multiplier and the numerator (e.g., ‘3|8 in’ rather than
‘3 * 1|8 in’). In some cases, this is not what is wanted; for example, the results for a cooking
recipe might show ‘3 * 1|2 cup’ as ‘3|2 cup’. With the ‘--show-factor’ option, a result
equivalent to 1.5 cups will display as ‘3 * 1|2 cup’ rather than ‘3|2 cup’. A user-specified
fractional unit with a numerator other than 1 is never overridden, however—if a unit list
specifies ‘3|4 cup;1|2 cup’, a result equivalent to 1 1/2 cups will always be shown as ‘2 *
3|4 cup’ whether or not the ‘--show-factor’ option is given.
--conformable
In non-interactive mode, show all units conformable with the original unit expression. Only one
unit expression is allowed; if you give more than one, units will exit with an error message and
return failure.
-v, --verbose
Give slightly more verbose output when converting units. When combined with the ‘-c’ option this
gives the same effect as ‘--check-verbose’. When combined with ‘--version’ produces a more
detailed output, equivalent to the ‘--info’ option.
-V, --version
Print the program version number, tell whether the readline library has been included, tell
whether UTF-8 support has been included; give the locale, the location of the default main units
data file, and the location of the personal units data file; indicate if the personal units data
file does not exist.
When given in combination with the ‘--terse’ option, the program prints only the version number
and exits.
When given in combination with the ‘--verbose’ option, the program, the ‘--version’ option has the
same effect as the ‘--info’ option below.
-I, --info
Print the information given with the ‘--version’ option, show the pathname of the units program,
show the status of the UNITSFILE and MYUNITSFILE environment variables, and additional information
about how units locates the related files. On systems running Microsoft Windows, the status of
the UNITSLOCALE environment variable and information about the related locale map are also given.
This option is usually of interest only to developers and administrators, but it can sometimes be
useful for troubleshooting.
Combining the ‘--version’ and ‘--verbose’ options has the same effect as giving ‘--info’.
-U, --unitsfile
Print the location of the default main units data file and exit; if the file cannot be found,
print “Units data file not found”.
-u units-system, --units units-system
Specify a CGS units system or natural units system. The supported units systems are: gauss[ian],
esu, emu, hlu, natural, natural-gauss, hartree, planck, planck-red, and si. See Alternative Unit
Systems for further information about these unit systems.
-l locale, --locale locale
Force a specified locale such as ‘en_GB’ to get British definitions by default. This overrides
the locale determined from system settings or environment variables. See Locale for a description
of locale format.
-n, --nolists
Disable conversion to unit lists.
-s, --strict
Suppress conversion of units to their reciprocal units. For example, units will normally convert
hertz to seconds because these units are reciprocals of each other. The strict option requires
that units be strictly conformable to perform a conversion, and will give an error if you attempt
to convert hertz to seconds.
-1, --one-line
Give only one line of output (the forward conversion); do not print the reverse conversion. If a
reciprocal conversion is performed, then units will still print the “reciprocal conversion” line.
-t, --terse
Print only a single conversion factor without any clutter, or if you request a definition, prints
just the definition (including its units). This option can be used when calling units from
another program so that the output is easy to parse. The command units --terse mile m produces
the output ‘1690.344’. This option has the combined effect of these options: ‘--strict’
‘--quiet’ ‘--one-line’ ‘--compact’. When combined with ‘--version’ it produces a display showing
only the program name and version number.
--compact
Give compact output featuring only the conversion factor; the multiplication and division signs
are not shown, and there is no leading whitespace. If you convert to a unit list, then the output
is a semicolon separated list of factors. This turns off the ‘--verbose’ option.
-q, --quiet, --silent
Suppress the display of statistics about the number of units loaded, any messages printed by the
units database, and the prompting of the user for units. This option does not affect how units
displays the results. This option is turned on by default if you invoke units with a unit
expression on the command line.
SCRIPTING WITH UNITS
Despite its numerous options, units cannot cover every conceivable unit-conversion task. For example,
suppose we have found some mysterious scale, but cannot figure out the units in which it is reporting.
We reach into our pocket, place a 3.75-gram coin on the scale, and observe the scale reading ‘0.120’.
How do we quickly determine the units? Or we might wonder if a unit has any “synonyms,” i.e., other
units with the same value.
The capabilities of units are easily extended with simple scripting. Both questions above involve
conformable units; on a system with Unix-like utilities, conversions to conformable units could be shown
accomplished with the following script:
#!/bin/sh
progname=`basename $0 .sh`
umsg="Usage: $progname [<number>] unit"
if [ $# -lt 1 ]
then
echo "$progname: missing quantity to convert"
echo "$umsg"
exit 1
fi
for unit in `units --conformable "$*" | cut -f 1 -d ' '`
do
echo "$*" # have -- quantity to convert
echo $unit # want -- conformable unit
done | units --terse --verbose
When units is invoked with no non-option arguments, it reads have/want pairs, on alternating lines, from
its standard input, so the task can be accomplished with only two invocations of units. This avoids the
computational overhead of needlessly reprocessing the units database for each conformable unit, as well
as the inherent system overhead of process invocation.
By itself, the script is not very useful. But it could be used in combination with other commands to
address specific tasks. For example, running the script through a simple output filter could help solve
the scale problem above. If the script is named conformable, running
$ conformable 3.75g | grep 0.120
gives
3.75g = 0.1205653 apounce
3.75g = 0.1205653 fineounce
3.75g = 0.1205653 ozt
3.75g = 0.1205653 tradewukiyeh
3.75g = 0.1205653 troyounce
So we might conclude that the scale is calibrated in troy ounces.
We might run
$ units --verbose are
Definition: 100 m^2 = 100 m^2
and wonder if ‘are’ has any synonyms, value. To find out, we could run
$ conformable are | grep "= 1 "
are = 1 a
are = 1 are
OUTPUT STYLES
The output can be tweaked in various ways using command line options. With no options, the output looks
like this
$ units
Currency exchange rates from FloatRates (USD base) on 2023-07-08
3612 units, 109 prefixes, 122 nonlinear units
You have: 23ft
You want: m
* 7.0104
/ 0.14264521
You have: m
You want: ft;in
3 ft + 3.3700787 in
This is arguably a bit cryptic; the ‘--verbose’ option makes clear what the output means:
$ units --verbose
Currency exchange rates from FloatRates (USD base) on 2023-07-08
3612 units, 109 prefixes, 122 nonlinear units
You have: 23 ft
You want: m
23 ft = 7.0104 m
23 ft = (1 / 0.14264521) m
You have: meter
You want: ft;in
meter = 3 ft + 3.3700787 in
The ‘--quiet’ option suppresses the clutter displayed when units starts, as well as the prompts to the
user. This option is enabled by default when you give units on the command line.
$ units --quiet
23 ft
m
* 7.0104
/ 0.14264521
$ units 23ft m
* 7.0104
/ 0.14264521
The remaining style options allow you to display only numerical values without the tab or the
multiplication and division signs, or to display just a single line showing the forward conversion:
$ units --compact 23ft m
7.0104
0.14264521
$ units --compact m 'ft;in'
3;3.3700787
$ units --one-line 23ft m
* 7.0104
$ units --one-line 23ft 1/m
reciprocal conversion
* 0.14264521
$ units --one-line 23ft kg
conformability error
7.0104 m
1 kg
Note that when converting to a unit list, the ‘--compact’ option displays a semicolon separated list of
results. Also be aware that the ‘one-line’ option doesn't live up to its name if you execute a
reciprocal conversion or if you get a conformability error. The former case can be prevented using the
‘--strict’ option, which suppresses reciprocal conversions. Similarly you can suppress unit list
conversion using ‘--nolists’. It is impossible to prevent the three line error output.
$ units --compact --nolists m 'ft;in'
Error in 'ft;in': Parse error
$ units --one-line --strict 23ft 1/m
The various style options can be combined appropriately. The ultimate combination is the ‘--terse’
option, which combines ‘--strict’, ‘--quiet’, ‘--one-line’, and ‘--compact’ to produce the minimal
output, just a single number for regular conversions and a semicolon separated list for conversion to
unit lists. This will likely be the best choice for programs that want to call units and then process
its result.
$ units --terse 23ft m
7.0104
$ units --terse m 'ft;in'
3;3.3700787
$ units --terse 23ft 1/m
conformability error
7.0104 m
1 / m
$ units --terse '1 mile'
1609.344 m
$ units --terse mile
5280 ft = 1609.344 m
ADDING YOUR OWN DEFINITIONS
Units Data Files
The units and prefixes that units can convert are defined in the units data file, typically
‘/usr/share/units/definitions.units’. If you can’t find this file, run units --version to get
information on the file locations for your installation. Although you can extend or modify this data
file if you have appropriate user privileges, it’s usually better to put extensions in separate files so
that the definitions will be preserved if you update units.
You can include additional data files in the units database using the ‘!include’ command in the standard
units data file. For example
!include /usr/local/share/units/local.units
might be appropriate for a site-wide supplemental data file. The location of the ‘!include’ statement in
the standard units data file is important; later definitions replace earlier ones, so any definitions in
an included file will override definitions before the ‘!include’ statement in the standard units data
file. With normal invocation, no warning is given about redefinitions; to ensure that you don’t have an
unintended redefinition, run units -c after making changes to any units data file.
If you want to add your own units in addition to or in place of standard or site-wide supplemental units
data files, you can include them in the ‘.units’ file in your home directory. If this file exists it is
read after the standard units data file, so that any definitions in this file will replace definitions of
the same units in the standard data file or in files included from the standard data file. This file
will not be read if any units files are specified on the command line. (Under Windows the personal units
file is named ‘unitdef.units’.) Running units -V will display the location and name of your personal
units file.
The units program first tries to determine your home directory from the HOME environment variable. On
systems running Microsoft Windows, if HOME does not exist, units attempts to find your home directory
from HOMEDRIVE, HOMEPATH and USERPROFILE. You can specify an arbitrary file as your personal units data
file with the MYUNITSFILE environment variable; if this variable exists, its value is used without
searching your home directory. The default units data files are described in more detail in Data Files.
Defining New Units and Prefixes
A unit is specified on a single line by giving its name and an equivalence. Comments start with a ‘#’
character, which can appear anywhere in a line. The backslash character (‘\’) acts as a continuation
character if it appears as the last character on a line, making it possible to spread definitions out
over several lines if desired. A file can be included by giving the command ‘!include’ followed by the
file’s name. The ‘!’ must be the first character on the line. The file will be sought in the same
directory as the parent file unless you give a full path. The name of the file to be included cannot
contain spaces or the comment character ‘#’.
Unit names cannot begin or end with an underscore (‘_’), a comma (‘,’) or a decimal point (‘.’). Names
must not contain any of the operator characters ‘+’, ‘-’, ‘*’, ‘/’, ‘|’, ‘^’, ‘;’, ‘~’, the comment
character ‘#’, or parentheses. To facilitate copying and pasting from documents, several typographical
characters are converted to operators: the figure dash (U+2012), minus (‘-’; U+2212), and en dash (‘–’;
U+2013) are converted to the operator ‘-’; the multiplication sign (‘×’; U+00D7), N-ary times operator
(U+2A09), dot operator (‘⋅’; U+22C5), and middle dot (‘·’; U+00B7) are converted to the operator ‘*’; the
division sign (‘÷’; U+00F7) is converted to the operator ‘/’; and the fraction slash (U+2044) is
converted to the operator ‘|’; accordingly, none of these characters can appear in unit names.
Names cannot begin with a digit, and if a name ends in a digit other than zero or one, the digit must be
preceded by a string beginning with an underscore, and afterwards consisting only of digits, decimal
points, or commas. For example, ‘foo_2’, ‘foo_2,1’, or ‘foo_3.14’ are valid names but ‘foo2’ or ‘foo_a2’
are invalid. The underscore is necessary because without it, units cannot determine whether ‘foo2’ is a
unit name or represents ‘foo^2’. Zero and one are exceptions because units never interprets them as
exponents.
You could define nitrous oxide as
N2O nitrogen 2 + oxygen
but would need to define nitrogen dioxide as
NO_2 nitrogen + oxygen 2
Be careful to define new units in terms of old ones so that a reduction leads to the primitive units,
which are marked with ‘!’ characters. Dimensionless units are indicated by using the string
‘!dimensionless’ for the unit definition.
When adding new units, be sure to use the ‘-c’ option to check that the new units reduce properly and
that there are no circular definitions that lead to endless loops. Because some errors may hide other
errors, you should run units with the ‘-c’ option again after correcting any errors, and keep doing so
until no errors are displayed.
If you define any units that contain ‘+’ characters in their definitions, carefully check them because
the ‘-c’ option will not catch non-conformable sums. Be careful with the ‘-’ operator as well. When
used as a binary operator, the ‘-’ character can perform addition or multiplication depending on the
options used to invoke units. To ensure consistent behavior use ‘-’ only as a unary negation operator
when writing units definitions. To multiply two units leave a space or use the ‘*’ operator with care,
recalling that it has two possible precedence values and may require parentheses to ensure consistent
behavior. To compute the difference of ‘foo’ and ‘bar’ write ‘foo+(-bar)’ or even ‘foo+-bar’.
You may wish to intentionally redefine a unit. When you do this, and use the ‘-c’ option, units displays
a warning message about the redefinition. You can suppress these warnings by redefining a unit using a
‘+’ at the beginning of the unit name. Do not include any white space between the ‘+’ and the redefined
unit name.
Here is an example of a short data file that defines some basic units:
m ! # The meter is a primitive unit
sec ! # The second is a primitive unit
rad !dimensionless # A dimensionless primitive unit
micro- 1e-6 # Define a prefix
minute 60 sec # A minute is 60 seconds
hour 60 min # An hour is 60 minutes
inch 72 m # Inch defined incorrectly terms of meters
ft 12 inches # The foot defined in terms of inches
mile 5280 ft # And the mile
+inch 0.0254 m # Correct redefinition, warning suppressed
A unit that ends with a ‘-’ character is a prefix. If a prefix definition contains any ‘/’ characters,
be sure they are protected by parentheses. If you define ‘half- 1/2’, then ‘halfmeter’ would be
equivalent to ‘1 / (2 meter)’.
Defining Nonlinear Units
Some unit conversions of interest are nonlinear; for example, temperature conversions between the
Fahrenheit and Celsius scales cannot be done by simply multiplying by conversion factors.
When you give a linear unit definition such as ‘inch 2.54 cm’ you are providing information that units
uses to convert values in inches into primitive units of meters. For nonlinear units, you give a
functional definition that provides the same information.
Nonlinear units are represented using a functional notation. It is best to regard this notation not as a
function call but as a way of adding units to a number, much the same way that writing a linear unit name
after a number adds units to that number. Internally, nonlinear units are defined by a pair of functions
that convert to and from linear units in the database, so that an eventual conversion to primitive units
is possible.
Here is an example nonlinear unit definition:
tempF(x) units=[1;K] domain=[-459.67,) range=[0,) \
(x+(-32)) degF + stdtemp ; (tempF+(-stdtemp))/degF + 32
A nonlinear unit definition comprises a unit name, a formal parameter name, two functions, and optional
specifications for units, the domain, and the range (the domain of the inverse function). The functions
tell units how to convert to and from the new unit. To produce valid results, the arguments of these
functions need to have the correct dimensions and be within the domains for which the functions are
defined.
The definition begins with the unit name followed immediately (with no spaces) by a ‘(’ character. In
the parentheses is the name of the formal parameter. Next is an optional specification of the units
required by the functions in the definition. In the example above, the ‘units=[1;K]’ specification
indicates that the ‘tempF’ function requires an input argument conformable with ‘1’ (i.e., the argument
is dimensionless), and that the inverse function requires an input argument conformable with ‘K’. For
normal nonlinear units definition, the forward function will always take a dimensionless argument; in
general, the inverse function will need units that match the quantity measured by your nonlinear unit.
Specifying the units enables units to perform error checking on function arguments, and also to assign
units to domain and range specifications, which are described later.
Next the function definitions appear. In the example above, the ‘tempF’ function is defined by
tempF(x) = (x+(-32)) degF + stdtemp
This gives a rule for converting ‘x’ in the units ‘tempF’ to linear units of absolute temperature, which
makes it possible to convert from tempF to other units.
To enable conversions to Fahrenheit, you must give a rule for the inverse conversions. The inverse will
be ‘x(tempF)’ and its definition appears after a ‘;’ character. In our example, the inverse is
x(tempF) = (tempF+(-stdtemp))/degF + 32
This inverse definition takes an absolute temperature as its argument and converts it to the Fahrenheit
temperature. The inverse can be omitted by leaving out the ‘;’ character and the inverse definition, but
then conversions to the unit will not be possible. If the inverse definition is omitted, the ‘--check’
option will display a warning. It is up to you to calculate and enter the correct inverse function to
obtain proper conversions; the ‘--check’ option tests the inverse at one point and prints an error if it
is not valid there, but this is not a guarantee that your inverse is correct.
With some definitions, the units may vary. For example, the definition
square(x) x^2
can have any arbitrary units, and can also take dimensionless arguments. In such a case, you should not
specify units. If a definition takes a root of its arguments, the definition is valid only for units
that yield such a root. For example,
squirt(x) sqrt(x)
is valid for a dimensionless argument, and for arguments with even powers of units.
Some definitions may not be valid for all real numbers. In such cases, units can handle errors better if
you specify an appropriate domain and range. You specify the domain and range as shown below:
baume(d) units=[1;g/cm^3] domain=[0,130.5] range=[1,10] \
(145/(145-d)) g/cm^3 ; (baume+-g/cm^3) 145 / baume
In this example the domain is specified after ‘domain=’ with the endpoints given in brackets. In accord
with mathematical convention, square brackets indicate a closed interval (one that includes its
endpoints), and parentheses indicate an open interval (one that does not include its endpoints). An
interval can be open or closed on one or both ends; an interval that is unbounded on either end is
indicated by omitting the limit on that end. For example, a quantity to which decibel (dB) is applied
may have any value greater than zero, so the range is indicated by ‘(0,)’:
decibel(x) units=[1;1] range=(0,) 10^(x/10); 10 log(decibel)
If the domain or range is given, the second endpoint must be greater than the first.
The domain and range specifications can appear independently and in any order along with the units
specification. The values for the domain and range endpoints are attached to the units given in the
units specification, and if necessary, the parameter value is adjusted for comparison with the endpoints.
For example, if a definition includes ‘units=[1;ft]’ and ‘range=[3,)’, the range will be taken as 3 ft to
infinity. If the function is passed a parameter of ‘900 mm’, that value will be adjusted to
2.9527559 ft, which is outside the specified range. If you omit the units specification from the
previous example, units can not tell whether you intend the lower endpoint to be 3 ft or 3 microfurlongs,
and can not adjust the parameter value of 900 mm for comparison. Without units, numerical values other
than zero or plus or minus infinity for domain or range endpoints are meaningless, and accordingly they
are not allowed. If you give other values without units, then the definition will be ignored and you
will get an error message.
Although the units, domain, and range specifications are optional, it’s best to give them when they are
applicable; doing so allows units to perform better error checking and give more helpful error messages.
Giving the domain and range also enables the ‘--check’ option to find a point in the domain to use for
its point check of your inverse definition.
You can make synonyms for nonlinear units by providing both the forward and inverse functions; inverse
functions can be obtained using the ‘~’ operator. So to create a synonym for ‘tempF’ you could write
fahrenheit(x) units=[1;K] tempF(x); ~tempF(fahrenheit)
This is useful for creating a nonlinear unit definition that differs slightly from an existing definition
without having to repeat the original functions. For example,
dBW(x) units=[1;W] range=[0,) dB(x) W ; ~dB(dBW/W)
If you wish a synonym to refer to an existing nonlinear unit without modification, you can do so more
simply by adding the synonym with appended parentheses as a new unit, with the existing nonlinear unit—
without parentheses—as the definition. So to create a synonym for ‘tempF’ you could write
fahrenheit() tempF
The definition must be a nonlinear unit; for example, the synonym
fahrenheit() meter
will result in an error message when units starts.
You may occasionally wish to define a function that operates on units. This can be done using a
nonlinear unit definition. For example, the definition below provides conversion between radius and the
area of a circle. This definition requires a length as input and produces an area as output, as
indicated by the ‘units=’ specification. Specifying the range as the nonnegative numbers can prevent
cryptic error messages.
circlearea(r) units=[m;m^2] range=[0,) pi r^2 ; sqrt(circlearea/pi)
Defining Piecewise Linear Units
Sometimes you may be interested in a piecewise linear unit such as many wire gauges. Piecewise linear
units can be defined by specifying conversions to linear units on a list of points. Conversion at other
points will be done by linear interpolation. A partial definition of zinc gauge is
zincgauge[in] 1 0.002, 10 0.02, 15 0.04, 19 0.06, 23 0.1
In this example, ‘zincgauge’ is the name of the piecewise linear unit. The definition of such a unit is
indicated by the embedded ‘[’ character. After the bracket, you should indicate the units to be attached
to the numbers in the table. No spaces can appear before the ‘]’ character, so a definition like ‘foo[kg
meters]’ is invalid; instead write ‘foo[kg*meters]’. The definition of the unit consists of a list of
pairs optionally separated by commas. This list defines a function for converting from the piecewise
linear unit to linear units. The first item in each pair is the function argument; the second item is
the value of the function at that argument (in the units specified in brackets). In this example, we
define ‘zincgauge’ at five points. For example, we set ‘zincgauge(1)’ equal to ‘0.002 in’. Definitions
like this may be more readable if written using continuation characters as
zincgauge[in] \
1 0.002 \
10 0.02 \
15 0.04 \
19 0.06 \
23 0.1
With the preceding definition, the following conversion can be performed:
You have: zincgauge(10)
You want: in
* 0.02
/ 50
You have: .01 inch
You want: zincgauge
5
If you define a piecewise linear unit that is not strictly monotonic, then the inverse will not be well
defined. If the inverse is requested for such a unit, units will return the smallest inverse.
After adding nonlinear units definitions, you should normally run ‘units --check’ to check for errors.
If the ‘units’ keyword is not given, the ‘--check’ option checks a nonlinear unit definition using a
dimensionless argument, and then checks using an arbitrary combination of units, as well as the square
and cube of that combination; a warning is given if any of these tests fail. For example,
Warning: function 'squirt(x)' defined as 'sqrt(x)'
failed for some test inputs:
squirt(7(kg K)^1): Unit not a root
squirt(7(kg K)^3): Unit not a root
Running ‘units --check’ will print a warning if a non-monotonic piecewise linear unit is encountered.
For example, the relationship between ANSI coated abrasive designation and mean particle size is non-
monotonic in the vicinity of 800 grit:
ansicoated[micron] \
. . .
600 10.55 \
800 11.5 \
1000 9.5 \
Running ‘units --check’ would give the error message
Table 'ansicoated' lacks unique inverse around entry 800
Although the inverse is not well defined in this region, it’s not really an error. Viewing such error
messages can be tedious, and if there are enough of them, they can distract from true errors. Error
checking for nonlinear unit definitions can be suppressed by giving the ‘noerror’ keyword; for the
examples above, this could be done as
squirt(x) noerror domain=[0,) range=[0,) sqrt(x); squirt^2
ansicoated[micron] noerror \
. . .
Use the ‘noerror’ keyword with caution. The safest approach after adding a nonlinear unit definition is
to run ‘units --check’ and confirm that there are no actual errors before adding the ‘noerror’ keyword.
Defining Unit List Aliases
Unit list aliases are treated differently from unit definitions, because they are a data entry shorthand
rather than a true definition for a new unit. A unit list alias definition begins with ‘!unitlist’ and
includes the alias and the definition; for example, the aliases included in the standard units data file
are
!unitlist hms hr;min;sec
!unitlist time year;day;hr;min;sec
!unitlist dms deg;arcmin;arcsec
!unitlist ftin ft;in;1|8 in
!unitlist usvol cup;3|4 cup;2|3 cup;1|2 cup;1|3 cup;1|4 cup;\
tbsp;tsp;1|2 tsp;1|4 tsp;1|8 tsp
Unit list aliases are only for unit lists, so the definition must include a ‘;’. Unit list aliases can
never be combined with units or other unit list aliases, so the definition of ‘time’ shown above could
not have been shortened to ‘year;day;hms’.
As usual, be sure to run ‘units --check’ to ensure that the units listed in unit list aliases are
conformable.
NUMERIC OUTPUT FORMAT
By default, units shows results to eight significant digits. You can change this with the
‘--exponential’, ‘--digits’, and ‘--output-format’ options. The first sets an exponential format (i.e.,
scientific notation) like that used in the original Unix units program, the second allows you to specify
a different number of significant digits, and the last allows you to control the output appearance using
the format for the printf function in the C programming language. If you only want to change the number
of significant digits or specify exponential format type, use the ‘--digits’ and ‘--exponential’ options.
The ‘--output-format’ option affords the greatest control of the output appearance, but requires at least
rudimentary knowledge of the printf format syntax. See Invoking Units for descriptions of these options.
Format Specification
The format specification recognized with the ‘--output-format’ option is a subset of that for printf.
The format specification has the form %[flags][width][.precision]type; it must begin with ‘%’, and must
end with a floating-point type specifier: ‘g’ or ‘G’ to specify the number of significant digits, ‘e’ or
‘E’ for scientific notation, and ‘f’ for fixed-point decimal. The ISO C99 standard added the ‘F’ type
for fixed-point decimal and the ‘a’ and ‘A’ types for hexadecimal floating point; these types are allowed
with compilers that support them. Type length modifiers (e.g., ‘L’ to indicate a long double) are
inapplicable and are not allowed.
The default format for units is ‘%.8g’; for greater precision, you could specify ‘-o %.15g’. The ‘g’ and
‘G’ format types use exponential format whenever the exponent would be less than -4, so the value
0.000013 displays as ‘1.3e-005’. These types also use exponential notation when the exponent is greater
than or equal to the precision, so with the default format, the value 5 × 10^7 displays as ‘50000000’ and
the value 5 × 10^8 displays as ‘5e+008’. If you prefer fixed-point display, you might specify ‘-o %.8f’;
however, small numbers will display very few significant digits, and values less than 5 × 10^-8 will show
nothing but zeros.
The format specification may include one or more optional flags: ‘+’, ‘ ’ (space), ‘#’, ‘-’, or ‘0’ (the
digit zero). The digit-grouping flag ‘'’ is allowed with compilers that support it. Flags are followed
by an optional value for the minimum field width, and an optional precision specification that begins
with a period (e.g., ‘.6’). The field width includes the digits, decimal point, the exponent, thousands
separators (with the digit-grouping flag), and the sign if any of these are shown.
Flags
The ‘+’ flag causes the output to have a sign (‘+’ or ‘-’). The space flag ‘ ’ is similar to the ‘+’
flag, except that when the value is positive, it is prefixed with a space rather than a plus sign; this
flag is ignored if the ‘+’ flag is also given. The ‘+’ or ‘ ’ flag could be useful if conversions might
include positive and negative results, and you wanted to align the decimal points in exponential
notation. The ‘#’ flag causes the output value to contain a decimal point in all cases; by default, the
output contains a decimal point only if there are digits (which can be trailing zeros) to the right of
the point. With the ‘g’ or ‘G’ types, the ‘#’ flag also prevents the suppression of trailing zeros. The
digit-grouping flag ‘'’ shows a thousands separator in digits to the left of the decimal point. This can
be useful when displaying large numbers in fixed-point decimal; for example, with the format ‘%f’,
You have: mile
You want: microfurlong
* 8000000.000000
/ 0.000000
the magnitude of the first result may not be immediately obvious without counting the digits to the left
of the decimal point. If the thousands separator is the comma (‘,’), the output with the format ‘%'f’
might be
You have: mile
You want: microfurlong
* 8,000,000.000000
/ 0.000000
making the magnitude readily apparent. Unfortunately, few compilers support the digit-grouping flag.
With the ‘-’ flag, the output value is left aligned within the specified field width. If a field width
greater than needed to show the output value is specified, the ‘0’ (zero) flag causes the output value to
be left padded with zeros until the specified field width is reached; for example, with the format
‘%011.6f’,
You have: troypound
You want: grain
* 5760.000000
/ 0000.000174
The ‘0’ flag has no effect if the ‘-’ (left align) flag is given.
Field Width
By default, the output value is left aligned and shown with the minimum width necessary for the specified
(or default) precision. If a field width greater than this is specified, the value shown is right
aligned, and padded on the left with enough spaces to provide the specified field width. A width
specification is typically used with fixed-point decimal to have columns of numbers align at the decimal
point; this arguably is less useful with units than with long columnar output, but it may nonetheless
assist in quickly assessing the relative magnitudes of results. For example, with the format ‘%12.6f’,
You have: km
You want: in
* 39370.078740
/ 0.000025
You have: km
You want: rod
* 198.838782
/ 0.005029
You have: km
You want: furlong
* 4.970970
/ 0.201168
Precision
The meaning of “precision” depends on the format type. With ‘g’ or ‘G’, it specifies the number of
significant digits (like the ‘--digits’ option); with ‘e’, ‘E’, ‘f’, or ‘F’, it specifies the maximum
number of digits to be shown after the decimal point.
With the ‘g’ and ‘G’ format types, trailing zeros are suppressed, so the results may sometimes have fewer
digits than the specified precision (as indicated above, the ‘#’ flag causes trailing zeros to be
displayed).
The default precision is 6, so ‘%g’ is equivalent to ‘%.6g’, and would show the output to six significant
digits. Similarly, ‘%e’ or ‘%f’ would show the output with six digits after the decimal point.
The C printf function allows a precision of arbitrary size, whether or not all of the digits are
meaningful. With most compilers, the maximum internal precision with units is 15 decimal digits (or 13
hexadecimal digits). With the ‘--digits’ option, you are limited to the maximum internal precision; with
the ‘--output-format’ option, you may specify a precision greater than this, but it may not be
meaningful. In some cases, specifying excess precision can result in rounding artifacts. For example, a
pound is exactly 7000 grains, but with the format ‘%.18g’, the output might be
You have: pound
You want: grain
* 6999.9999999999991
/ 0.00014285714285714287
With the format ‘%.25g’ you might get the following:
You have: 1/3
You want:
Definition: 0.333333333333333314829616256247
In this case the displayed value includes a series of digits that represent the underlying binary
floating-point approximation to 1/3 but are not meaningful for the desired computation. In general, the
result with excess precision is system dependent. The precision affects only the display of numbers; if
a result relies on physical constants that are not known to the specified precision, the number of
physically meaningful digits may be less than the number of digits shown.
See the documentation for printf for more detailed descriptions of the format specification.
The ‘--output-format’ option is incompatible with the ‘--exponential’ or ‘--digits’ options; if the
former is given in combination with either of the latter, the format is controlled by the last option
given.
LOCALIZATION
Some units have different values in different locations. The localization feature accommodates this by
allowing a units data file to specify definitions that depend on the user’s locale.
Locale
A locale is a subset of a user’s environment that indicates the user’s language and country, and some
attendant preferences, such as the formatting of dates. The units program attempts to determine the
locale from the POSIX setlocale function; if this cannot be done, units examines the environment
variables LC_CTYPE and LANG. On POSIX systems, a locale is of the form language_country, where language
is the two-character code from ISO 639-1 and country is the two-character code from ISO 3166-1; language
is lower case and country is upper case. For example, the POSIX locale for the United Kingdom is en_GB.
On systems running Microsoft Windows, the value returned by setlocale is different from that on POSIX
systems; units attempts to map the Windows value to a POSIX value by means of a table in the file
‘locale_map.txt’ in the same directory as the other data files. The file includes entries for many
combinations of language and country, and can be extended to include other combinations. The
‘locale_map.txt’ file comprises two tab-separated columns; each entry is of the form
Windows-locale POSIX-locale
where POSIX-locale is as described above, and Windows-locale typically spells out both the language and
country. For example, the entry for the United States is
English_United States en_US
You can force units to run in a desired locale by using the ‘-l’ option.
In order to create unit definitions for a particular locale you begin a block of definitions in a unit
datafile with ‘!locale’ followed by a locale name. The ‘!’ must be the first character on the line. The
units program reads the following definitions only if the current locale matches. You end the block of
localized units with ‘!endlocale’. Here is an example, which defines the British gallon.
!locale en_GB
gallon 4.54609 liter
!endlocale
Additional Localization
Sometimes the locale isn’t sufficient to determine unit preferences. There could be regional
preferences, or a company could have specific preferences. Though probably uncommon, such differences
could arise with the choice of English customary units outside of English-speaking countries. To address
this, units allows specifying definitions that depend on environment variable settings. The environment
variables can be controlled based on the current locale, or the user can set them to force a particular
group of definitions.
A conditional block of definitions in a units data file begins with either ‘!var’ or ‘!varnot’ following
by an environment variable name and then a space separated list of values. The leading ‘!’ must appear
in the first column of a units data file, and the conditional block is terminated by ‘!endvar’.
Definitions in blocks beginning with ‘!var’ are executed only if the environment variable is exactly
equal to one of the listed values. Definitions in blocks beginning with ‘!varnot’ are executed only if
the environment variable does not equal any of the list values.
The inch has long been a customary measure of length in many places. The word comes from the Latin uncia
meaning “one twelfth,” referring to its relationship with the foot. By the 20th century, the inch was
officially defined in English-speaking countries relative to the yard, but until 1959, the yard differed
slightly among those countries. In France the customary inch, which was displaced in 1799 by the meter,
had a different length based on a french foot. These customary definitions could be accommodated as
follows:
!var INCH_UNIT usa
yard 3600|3937 m
!endvar
!var INCH_UNIT canada
yard 0.9144 meter
!endvar
!var INCH_UNIT uk
yard 0.91439841 meter
!endvar
!var INCH_UNIT canada uk usa
foot 1|3 yard
inch 1|12 foot
!endvar
!var INCH_UNIT france
foot 144|443.296 m
inch 1|12 foot
line 1|12 inch
!endvar
!varnot INCH_UNIT usa uk france canada
!message Unknown value for INCH_UNIT
!endvar
When units reads the above definitions it will check the environment variable INCH_UNIT and load only the
definitions for the appropriate section. If INCH_UNIT is unset or is not set to one of the four values
listed, then units will run the last block. In this case that block uses the ‘!message’ command to
display a warning message. Alternatively that block could set default values.
In order to create default values that are overridden by user settings the data file can use the ‘!set’
command, which sets an environment variable only if it is not already set; these settings are only for
the current units invocation and do not persist. So if the example above were preceded by ‘!set
INCH_UNIT france’, then this would make ‘france’ the default value for INCH_UNIT. If the user had set
the variable in the environment before invoking units, then units would use the user’s value.
To link these settings to the user’s locale you combine the ‘!set’ command with the ‘!locale’ command.
If you wanted to combine the above example with suitable locales you could do by preceding the above
definition with the following:
!locale en_US
!set INCH_UNIT usa
!endlocale
!locale en_GB
!set INCH_UNIT uk
!endlocale
!locale en_CA
!set INCH_UNIT canada
!endlocale
!locale fr_FR
!set INCH_UNIT france
!endlocale
!set INCH_UNIT france
These definitions set the overall default for INCH_UNIT to ‘france’ and set default values for four
locales appropriately. The overall default setting comes last so that it only applies when INCH_UNIT was
not set by one of the other commands or by the user.
If the variable given after ‘!var’ or ‘!varnot’ is undefined, then units prints an error message and
ignores the definitions that follow. Use ‘!set’ to create defaults to prevent this situation from
arising. The ‘-c’ option only checks the definitions that are active for the current environment and
locale, so when adding new definitions take care to check that all cases give rise to a well defined set
of definitions.
ENVIRONMENT VARIABLES
The units program uses the following environment variables:
HOME Specifies the location of your home directory; it is used by units to find a personal units data
file ‘.units’. On systems running Microsoft Windows, the file is ‘unitdef.units’, and if HOME
does not exist, units tries to determine your home directory from the HOMEDRIVE and HOMEPATH
environment variables; if these variables do not exist, units finally tries USERPROFILE—typically
‘C:\Users\username’ (Windows Vista and Windows 7) or ‘C:\Documents and Settings\username’
(Windows XP).
LC_CTYPE, LANG
Checked to determine the locale if units cannot obtain it from the operating system. Sections of
the default main units data file are specific to certain locales.
MYUNITSFILE
Specifies your personal units data file. If this variable exists, units uses its value rather
than searching your home directory for ‘.units’. The personal units file will not be loaded if
any data files are given using the ‘-f’ option.
PAGER Specifies the pager to use for help and for displaying the conformable units. The help function
browses the units database and calls the pager using the ‘+n’n syntax for specifying a line
number. The default pager is more; PAGER can be used to specify alternatives such as less, pg,
emacs, or vi.
UNITS_ENGLISH
Set to either ‘US’ or ‘GB’ to choose United States or British volume definitions, overriding the
default from your locale.
UNITSFILE
Specifies the units data file to use (instead of the default). You can only specify a single
units data file using this environment variable. If units data files are given using the ‘-f’
option, the file specified by UNITSFILE will be not be loaded unless the ‘-f’ option is given with
the empty string (‘units -f ""’).
UNITSLOCALEMAP
Windows only; this variable has no effect on Unix-like systems. Specifies the units locale map
file to use (instead of the default). This variable seldom needs to be set, but you can use it to
ensure that the locale map file will be found if you specify a location for the units data file
using either the ‘-f’ option or the UNITSFILE environment variable, and that location does not
also contain the locale map file.
UNITS_SYSTEM
This environment variable is used in the default main data file to select CGS measurement systems.
Currently supported systems are ‘esu’, ‘emu’, ‘gauss[ian]’, ‘hlu’, ‘natural’, ‘natural-gauss’,
‘planck’, ‘planck-red’, ‘hartree’ and ‘si’. The default is ‘si’.
DATA FILES
The units program uses four default data files: the main data file, ‘definitions.units’; the atomic
masses of the elements, ‘elements.units’; currency exchange rates, ‘currency.units’, and the US Consumer
Price Index, ‘cpi.units’. The last three files are loaded by means of ‘!include’ directives in the main
file (see Database Command Syntax). The program can also use an optional personal units data file
‘.units’ (‘unitdef.units’ under Windows) located in the user’s home directory. The personal units data
file is described in more detail in Units Data Files.
On Unix-like systems, the data files are typically located in ‘/usr/share/units’ if units is provided
with the operating system, or in ‘/usr/local/share/units’ if units is compiled from the source
distribution. Note that the currency file ‘currency.units’ is a symbolic link to another location.
On systems running Microsoft Windows, the files may be in the same locations if Unix-like commands are
available, a Unix-like file structure is present (e.g., ‘C:/usr/local’), and units is compiled from the
source distribution. If Unix-like commands are not available, a more common location is
‘C:\Program Files (x86)\GNU\units’ (for 64-bit Windows installations) or ‘C:\Program Files\GNU\units’
(for 32-bit installations).
If units is obtained from the GNU Win32 Project (http://gnuwin32.sourceforge.net/), the files are
commonly in ‘C:\Program Files\GnuWin32\share\units’.
If the default main units data file is not an absolute pathname, units will look for the file in the
directory that contains the units program; if the file is not found there, units will look in a directory
../share/units relative to the directory with the units program.
You can determine the location of the files by running ‘units --version’. Running ‘units --info’ will
give you additional information about the files, how units will attempt to find them, and the status of
the related environment variables.
UNICODE SUPPORT
The standard units data file is in Unicode, using UTF-8 encoding. Most definitions use only ASCII
characters (i.e., code points U+0000 through U+007F); definitions using non-ASCII characters appear in
blocks beginning with ‘!utf8’ and ending with ‘!endutf8’.
The non-ASCII definitions are loaded only if the platform and the locale support UTF-8. Platform support
is determined when units is compiled; the locale is checked at every invocation of units. To see if your
version of units includes Unicode support, invoke the program with the ‘--version’ option.
When Unicode support is available, units checks every line within UTF-8 blocks in all of the units data
files for invalid or non-printing UTF-8 sequences; if such sequences occur, units ignores the entire
line. In addition to checking validity, units determines the display width of non-ASCII characters to
ensure proper positioning of the pointer in some error messages and to align columns for the ‘search’ and
‘?’ commands.
As of early 2019, Microsoft Windows provides limited support for UTF-8 in console applications, and
accordingly, units does not support Unicode on Windows. The UTF-16 and UTF-32 encodings are not
supported on any platforms.
If Unicode support is available and definitions that contain non-ASCII UTF-8 characters are added to a
units data file, those definitions should be enclosed within ‘!utf8’ ... ‘!endutf8’ to ensure that they
are only loaded when Unicode support is available. As usual, the ‘!’ must appear as the first character
on the line. As discussed in Units Data Files, it’s usually best to put such definitions in supplemental
data files linked by an ‘!include’ command or in a personal units data file.
When Unicode support is not available, units makes no assumptions about character encoding, except that
characters in the range 00–7F hexadecimal correspond to ASCII encoding. Non-ASCII characters are simply
sequences of bytes, and have no special meanings; for definitions in supplementary units data files, you
can use any encoding consistent with this assumption. For example, if you wish to use non-ASCII
characters in definitions when running units under Windows, you can use a character set such as Windows
“ANSI” (code page 1252 in the US and Western Europe); if this is done, the console code page must be set
to the same encoding for the characters to display properly. You can even use UTF-8, though some
messages may be improperly aligned, and units will not detect invalid UTF-8 sequences. If you use UTF-8
encoding when Unicode support is not available, you should place any definitions with non-ASCII
characters outside ‘!utf8’ ... ‘!endutf8’ blocks—otherwise, they will be ignored.
Except for code examples, typeset material usually uses the Unicode symbols for mathematical operators.
To facilitate copying and pasting from such sources, several typographical characters are converted to
the ASCII operators used in units: the figure dash (U+2012), minus (‘-’; U+2212), and en dash (‘–’;
U+2013) are converted to the operator ‘-’; the multiplication sign (‘×’; U+00D7), N-ary times operator
(U+2A09), dot operator (‘⋅’; U+22C5), and middle dot (‘·’; U+00B7) are converted to the operator ‘*’; the
division sign (‘÷’; U+00F7) is converted to the operator ‘/’; and the fraction slash (U+2044) is
converted to the operator ‘|’.
READLINE SUPPORT
If the readline package has been compiled in, then when units is used interactively, numerous command
line editing features are available. To check if your version of units includes readline, invoke the
program with the ‘--version’ option.
For complete information about readline, consult the documentation for the readline package. Without any
configuration, units will allow editing in the style of emacs. Of particular use with units are the
completion commands.
If you type a few characters and then hit ESC followed by ?, then units will display a list of all the
units that start with the characters typed. For example, if you type metr and then request completion,
you will see something like this:
You have: metr
metre metriccup metrichorsepower metrictenth
metretes metricfifth metricounce metricton
metriccarat metricgrain metricquart metricyarncount
You have: metr
If there is a unique way to complete a unit name, you can hit the TAB key and units will provide the rest
of the unit name. If units beeps, it means that there is no unique completion. Pressing the TAB key a
second time will print the list of all completions.
The readline library also keeps a history of the values you enter. You can move through this history
using the up and down arrows. The history is saved to the file ‘.units_history’ in your home directory
so that it will persist across multiple units invocations. If you wish to keep work for a certain
project separate you can change the history filename using the ‘--history’ option. You could, for
example, make an alias for units to units --history .units_history so that units would save separate
history in the current directory. The length of each history file is limited to 5000 lines. Note also
that if you run several concurrent copies of units each one will save its new history to the history file
upon exit.
UPDATING CURRENCY EXCHANGE RATES AND CPI
Currency Exchange Rates
The units program database includes currency exchange rates and prices for some precious metals. Of
course, these values change over time, sometimes very rapidly, and units cannot provide real-time values.
To update the exchange rates, run units_cur, which rewrites the file containing the currency rates,
typically ‘/var/lib/units/currency.units’ or ‘/usr/local/com/units/currency.units’ on a Unix-like system
or ‘C:\Program Files (x86)\GNU\units\definitions.units’ on a Windows system.
This program requires Python 3 (https://www.python.org). The program must be run with suitable
permissions to write the file. To keep the rates updated automatically, run it using a cron job on a
Unix-like system, or a similar scheduling program on a different system.
Reliable free sources of currency exchange rates have been annoyingly ephemeral. The program currently
supports several sources:
• ExchangeRate-API.com (https://www.exchangerate-api.com).
The default currency server. Allows open access without an API key, with unlimited API requests.
Rates update once a day, the US dollar (‘USD’) is the default base currency, and you can choose your
base currency with the ‘-b’ option described below. You can optionally sign up for an API key to
access paid benefits such as faster data update rates.
• FloatRates (https://www/floatrates.com).
The US dollar (‘USD’) is the default base currency. You can change the base currency with the ‘-b’
option described below. Allowable base currencies are listed on the FloatRates website. Exchange
rates update daily.
• The European Central Bank (https://www.ecb.europa.eu).
The base currency is always the euro (‘EUR’). Exchange rates update daily. This source offers a
more limited list of currencies than the others.
• Fixer (https://fixer.io).
Registration for a free API key is required. With a free API key, base currency is the euro;
exchange rates are updated hourly, the service has a limit of 1,000 API calls per month, and SSL
encryption (https protocol) is not available. Most of these restrictions are eliminated or reduced
with paid plans.
• open exchange rates (https://openexchangerates.org).
Registration for a free API key is required. With a free API key, the base currency is the US
dollar; exchange rates are updated hourly, and there is a limit of 1,000 API calls per month. Most
of these restrictions are eliminated or reduced with paid plans.
The default source is FloatRates; you can select a different one using ‘-s’ option described below.
Precious metals pricing is obtained from Packetizer (www.packetizer.com). This site updates once per
day.
US Consumer Price Index
The units program includes the US Consumer Price Index (CPI) published by the US Bureau of Labor
Statistics: specifically, the Consumer Price Index for All Urban Consumers (CPI-U), Series CUUR0000SA0.
The units_cur command updates the CPI and saves the result in ‘cpi.units’ in the same location as
‘currency.units’. The data are obtained via the BLS Public Data API (https://www.bls.gov/developers/).
This data updates once a month. When units_cur runs it will only attempt to update the CPI data if the
current CPI data file is from a previous month, or if the current date is after the 18th of the month.
Invoking units_cur
You invoke units_cur like this:
units_cur [options] [currency_file] [cpi_file]
By default, the output is written to the default currency and CPI files described above; this is usually
what you want, because this is where units looks for the files. If you wish, you can specify different
filenames on the command line and units_cur will write the data to those files. If you give ‘-’ for a
file it will write to standard output.
The following options are available:
-h, --help
Print a summary of the options for units_cur.
-V, --version
Print the units_cur version number.
-v, --verbose
Give slightly more verbose output when attempting to update currency exchange rates.
-s source, --source source
Specify the source for currency exchange rates; currently supported values are ‘floatrates’ (for
FloatRates), ‘eubank’ (for the European Central Bank), ‘fixer’ (for Fixer), and
‘openexchangerates’ (for open exchange rates); the last two require an API key to be given with
the ‘-k’ option.
-b base, --base base
Set the base currency (when allowed by the site providing the data). base should be a 3-letter
ISO currency code, e.g., ‘USD’. The specified currency will be the primitive currency unit used
by units. You may find it convenient to specify your local currency. Conversions may be more
accurate and you will be able to convert to your currency by simply hitting Enter at the
‘You want:’ prompt. This option is ignored if the source does not allow specifying the base
currency. (Currently only floatrates supports this option.)
-k key, --key key
Set the API key to key for currency sources that require it.
--blskey BLSkey
Set the US Bureau of Labor Statistics (BLS) key for fetching CPI data. Without a BLS key you
should be able to fetch the CPI data exactly one time per day. If you want to use a key you must
request a personal key from BLS.
DATABASE COMMAND SYNTAX
unit definition
Define a regular unit.
prefix- definition
Define a prefix.
funcname(var) noerror units=[in-units,out-units] domain=[x1,x2] range=[y1,y2] definition(var) ;
inverse(funcname)
Define a nonlinear unit or unit function. The four optional keywords noerror, ‘units=’, ‘range=’
and ‘domain=’ can appear in any order. The definition of the inverse is optional.
tabname[out-units] noerror pair-list
Define a piecewise linear unit. The pair list gives the points on the table listed in ascending
order. The noerror keyword is optional.
!endlocale
End a block of definitions beginning with ‘!locale’
!endutf8
End a block of definitions begun with ‘!utf8’
!endvar
End a block of definitions begun with ‘!var’ or ‘!varnot’
!include file
Include the specified file.
!locale value
Load the following definitions only of the locale is set to value.
!message text
Display text when the database is read unless the quiet option (‘-q’) is enabled. If you omit
text, then units will display a blank line. Messages will also appear in the log file.
!prompt text
Prefix the ‘You have:’ prompt with the specified text. If you omit text, then any existing prefix
is canceled.
!set variable value
Sets the environment variable, variable, to the specified value only if it is not already set.
!unitlist alias definition
Define a unit list alias.
!utf8 Load the following definitions only if units is running with UTF-8 enabled.
!var envar value-list
Load the block of definitions that follows only if the environment variable envar is set to one of
the values listed in the space-separated value list. If envar is not set, units prints an error
message and ignores the block of definitions.
!varnot envar value-list
Load the block of definitions that follows only if the environment variable envar is set to value
that is not listed in the space-separated value list. If envar is not set, units prints an error
message and ignores the block of definitions.
FILES
/usr/share/units/definitions.units — the standard units data file
AUTHOR
units was written by Adrian Mariano
16 February 2024 UNITS(1)