UNITS(1) General Commands Manual UNITS(1)

units — conversion calculator

units [-ehqtUVv] [-f unitsfile] [-o format] [from to]

The units program converts quantities expressed in various scales to their equivalents in other scales. It can only handle multiplicative or affine scale changes.

units can work interactively by prompting the user for input (see EXAMPLES) or non-interactively, providing a conversion for given arguments from and to.

The following options are available:

, --exponential
Same as -o %6e (see the description of the -o flag).
unitsfile, --file unitsfile
Specify the name of the units data file to load. This option may be specified multiple times.
historyfile, --history historyfile
Ignored, for compatibility with GNU units.
, --help
Show an overview of options.
format, --output-format format
Select the output format string by which numbers are printed. Defaults to “%.8g”.
, --quiet
Suppress prompting of the user for units and the display of statistics about the number of units loaded.
, --terse
Only print the result. This is used when calling units from other programs for easy to parse results.
, --unitsfile
Print the location of the default unit file if it exists. Otherwise, print an error message.
, --version
Print the version number (which is fixed at “FreeBSD units”), the path to the units data file and exit.
, --verbose
Print the units in the conversion output. Be more verbose in general.
from to
Allow a single unit conversion to be done directly from the command line. The program will not print prompts. It will print out the result of the single specified conversion. Both arguments, i.e., from and to, can be just a unit (e.g., “cm”), a quantity (e.g., “42”), or a quantity with a unit (e.g., “42 cm”)

  • Powers of units can be specified using the “^” character as shown in the example, or by simple concatenation: “cm3” is equivalent to “cm^3”. See the BUGS section for details on the limitations of exponent values.
  • Multiplication of units can be specified by using spaces (“ ”), a dash (“-”) or an asterisk (“*”).
  • Division of units is indicated by the slash (“/”).
  • Division of numbers must be indicated using the vertical bar (“|”).

Note that multiplication has a higher precedence than division, so “m/s/s” is the same as “m/s^2” or “m/s s”.

The conversion information is read from a units data file. The default file includes definitions for most familiar units, abbreviations and metric prefixes. Some constants of nature included are:

pi ratio of circumference to diameter
c speed of light
e charge on an electron
g acceleration of gravity
force same as g
mole Avogadro's number
water pressure per unit height of water
mercury pressure per unit height of mercury
au astronomical unit

The unit “pound” is a unit of mass. Compound names are run together so “pound force” is a unit of force. The unit “ounce” is also a unit of mass. The fluid ounce is “floz”. British units that differ from their US counterparts are prefixed with “br”, and 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 units will try to remove a trailing “s” or a trailing “es” and check again for a match.

To find out what units are available read the standard units file. If you want to add your own units you can supply your own file. A unit is specified on a single line by giving its name and an equivalence. 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. The units program will not detect infinite loops that could be caused by careless unit definitions. Comments in the unit definition file begin with a “#” or “/” character at the beginning of a line.

Prefixes are defined in the same way as standard units, but with a trailing dash (“-”) at the end of the prefix name. If a unit is not found even after removing trailing “s” or “es”, then it will be checked against the list of prefixes. Prefixes will be removed until a legal base unit is identified.

The colon-separated list of root directories at which units tries to find /usr/share/misc/definitions.units.

For example if PATH is set to “/tmp:/:/usr/local”, no -f flags are provided, and /usr/share/misc/definitions.units is missing then units tries to open the following files as the default units file: /tmp/usr/share/misc/definitions.units, /usr/share/misc/definitions.units, and /usr/local/usr/share/misc/definitions.units.

/usr/share/misc/definitions.units
The standard units file.

The units utility exits 0 on success, and >0 if an error occurs.

: Simple conversion of units

This example shows how to do simple conversions, for example from gigabytes to bytes:

$ units -o %0.f -t '4 gigabytes' bytes
4294967296

The -o %0.f part of the command is required to print the result in a non-scientific notation (e.g, 4294967296 instead of 4.29497e+09).

: Interactive usage

Here is an example of an interactive session where the user is prompted for units:

You have: meters
You want: feet
        * 3.2808399
        / 0.3048

You have: cm^3
You want: gallons
        * 0.00026417205
        / 3785.4118

You have: meters/s
You want: furlongs/fortnight
        * 6012.8848
        / 0.00016630952

You have: 1|2 inch
You want: cm
        * 1.27
        / 0.78740157

You have: 85 degF
You want: degC
        29.444444
: Difference between “|” and “/” division

The following command shows how to convert half a meter to centimeters.

$ units '1|2 meter' cm
        * 50
        / 0.02

units prints the expected result because the division operator for numbers (“|”) was used.

Using the division operator for units (“/”) would result in an error:

$ units '1/2 meter' cm
conformability error
        0.5 / m
        0.01 m

It is because units interprets “1/2 meter” as “0.5/meter”, which is not conformable to “cm”.

: Simple units file
Here is an example of a short units file that defines some basic units:

m !a!
sec !b!
micro- 1e-6
minute 60 sec
hour 60 min
inch 0.0254 m
ft 12 inches
mile 5280 ft
: Viewing units and conversions of the default units file
The following shell one-liner allows the user to view the contents of the default units file: 
$ less "$(units -U)"

can't find units file '%s'
The default units file is not in its default location (see FILES) and it is not present in any file tree starting with their roots at directories from PATH (see ENVIRONMENT).
cap_rights_limit() failed
See capsicum(4).
conformability error
It is not possible to reduce the given units to one common unit: they are not conformable. Instead of a conversion, units will display the reduced form for each provided 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
Could not initialize history
See editline(3).
dupstr
strdup(3) failed.
memory for prefixes exceeded in line %d
Over 100 prefixes were defined.
memory for units exceeded in line %d
Over 1000 prefixes were defined.
memory overflow in unit reduction
The requested conversion involves too many units (see BUGS).
redefinition of prefix '%s' on line %d ignored
redefinition of unit '%s' on line %d ignored
unexpected end of prefix on line %d
unexpected end of unit on line %d
Units data file not found
The default units file is missing.
unable to enter capability mode
See capsicum(4).
unable to open units file '%s'
One of the user-specified units files cannot be opened.
unit reduces to zero
unknown unit '%s'
The provided unit cannot be found in the units file.
WARNING: conversion of non-proportional quantities.
units may fail to convert from to to because the units are not proportional. The warning is printed when a quantity is a part of the to argument. It can be illustrated on an example of conversion from Fahrenheit to Celsius: 
$ units "degF" "degC"
         (-> x*0.55555556g -17.777778g)
         (<- y*1.8g 32g)
$ units "degF" "1 degC"
WARNING: conversion of non-proportional quantities.
         (-> x*0.55555556g -17.777778g)
         (<- y*1.8g 32g)
$ units "1 degF" "1 degC"
WARNING: conversion of non-proportional quantities.
        -17.222222

bc(1)

The units first appeared in NetBSD and was ported to FreeBSD 2.2.0.

The manual page was significantly rewritten in FreeBSD 13.0 by Mateusz Piotrowski <0mp@FreeBSD.org>.

Adrian Mariano <adrian@cam.cornell.edu>

The effect of including a “/” in a prefix is surprising.

Exponents entered by the user can be only one digit. You can work around this by multiplying several terms.

The user must use “|” to indicate division of numbers and “/” to indicate division of symbols. This distinction should not be necessary.

The program contains various arbitrary limits on the length of the units converted and on the length of the data file.

The program should use a hash table to store units so that it does not take so long to load the units list and check for duplication.

It is not possible to convert a negative value.

The units program does not handle reductions of long lists of units very well:

$ units "$(yes m | head -n 154)" "$(yes cm | head -n 154)"
        * 1e+308
        / 1e-308
$ units "$(yes m | head -n 333)" "$(yes cm | head -n 333)"
        * inf
        / 0
$ units "$(yes m | head -n 500)" "$(yes cm | head -n 500)"
units: memory overflow in unit reduction
conformability error
        1 m^500
        1 centi cm^499
$ units "$(yes m | head -n 501)" "$(yes cm | head -n 501)"
units: memory overflow in unit reduction
units: memory overflow in unit reduction
units: memory overflow in unit reduction
conformability error
        1 m^500
        1 centi cm^499
March 17, 2020 macOS 15.2