|Stable release||2.13 / June 21, 2016|
|Operating system||Linux, FreeBSD, NetBSD, OpenBSD, most Unix implementations, Windows|
|Platform||Included with some Linux distributions. Two Windows binary distributions are available; distribution for other platforms is source only.|
GNU Units is a cross-platform computer program for conversion of units of quantities. It has a database of measurement units, including esoteric and historical units. This for instance allows conversion of velocities specified in furlongs per fortnight, and pressures specified in tons per acre. Output units are checked for consistency with the input, allowing verification of conversion of complex expressions.
GNU Units was written by Adrian Mariano as an implementation of the units utility included with the Unix operating system. It was originally available under a permissive license. The GNU variant is distributed under the GPL although the FreeBSD project maintains a free fork of units from before the license change.
units (Unix utility)
The original units program has been a standard part of Unix since the early Bell Laboratories versions. Source code for a version very similar to the original is available from the Heirloom Project. 
The GNU implementation
GNU units includes several extensions to the original version, including
- Exponents can be written with
- Exponents can be larger than 9 if written with
- Rational and decimal exponents are supported.
- Sums of units (e.g.,
btu + ft lbf) can be converted.
- Conversions can be made to sums of units, termed unit lists (e.g., from degrees to degrees, minutes, and seconds).
- Units that measure reciprocal dimensions can be converted (e.g.,
- Parentheses for grouping are supported. This sometimes allows more natural expressions, such as in the example given in Complex units expressions.
- Roots of units (e.g.,
sqrt((lbf/inch) / lb)can be computed.
- Nonlinear units conversions (e.g., °F to °C) are supported.
- Functions such as sin, cos, ln, log, and log2 are included.
- A script for updating the currency conversions is included; the script requires Python.
Units definitions, including nonlinear conversions and unit lists, are user extensible.
The plain text database definitions.units is a good reference in itself, as it is extensively commented and cites numerous sources.
UDUNITS is a similar utility program, except that it has an additional programming library interface and date conversion abilities. UDUNITS is considered the de facto program and library for variable unit conversion for netCDF files.
The latest version of GNU Units, 2.13, was released on June 21, 2016; it fixed several minor bugs and improved support for building on Windows. Version 2.10, released on March 26, 2014, added support for rational exponents greater than one, and added the ability to save an interactive session in a file to provide a record of the conversions performed. Beginning with version 2.10, a 32-bit Windows binary distribution has been available on the project Web page (a 32-bit Windows port of version 1.87 has been available since 2008 as part of the GnuWin32 project).
Version 2.02, released on July 11, 2013, added hexadecimal floating-point output and two other options to simplify changing the output format.
Version 2.0, released on July 2, 2012, added the ability to convert to sums of units, such as hours and minutes or feet and inches. In addition, this release added support for UTF-8 encoding. Provision for locale-specific unit definitions was added. The syntax for defining non-linear units was changed, and added optional domain and range specifications. The names of the standard and personal units data files were changed, and the currency definitions were placed in a separate data file; a Python script for updating the currency definitions was added.
The version history is covered in detail in the NEWS file included with the source distribution.
Units will output the result of the conversion in two lines. Usually, the first line (multiplication) is the desired result; the second line is the same conversion expressed as a division.
If you attempt to convert types of measurements that are incompatible, units will print a conformability error message and display a reduced form of each measurement.
The examples that follow show results from GNU units version 2.10.
Currency exchange rates from www.timegenie.com on 2014-03-28 2729 units, 92 prefixes, 77 nonlinear units You have: 10 furlongs You want: miles * 1.25 / 0.8 You have: 1 gallon + 3 pints You want: quarts * 5.5 / 0.18181818 You have: sqrt(meter) ^ Unit not a root You have: sqrt(acre) You want: ft * 208.71033 / 0.0047913298 You have: 21 btu + 6500 ft lbf You want: btu * 29.352939 / 0.034068139 You have: _ You want: J * 30968.99 / 3.2290366e-005 You have: 3.277 hr You want: time 3 hr + 16 min + 37.2 sec
The underscore ('_') is used to indicate the result of the last successful unit conversion.
On the command line (non-interactive)
C:\>units "ten furlongs per fortnight" "kilometers per hour" * 0.0059871429 / 167.02458
% units cup ounces conformability error 0.00023658824 m^3 0.028349523 kg
Complex units expressions
where ΔP is the pressure drop, ρ 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. It might be desirable to have the equation in the form
that would accept typical US units; the constant A1 could be determined manually using the unit-factor method, but it could be determined more quickly and easily using units:
$ units "(8/pi^2)(lbm/ft^3)ft(ft^3/s)^2(1/in^5)" psi * 43.533969 / 0.022970568
Crane Technical Paper No. 410, Eq. 3-5, gives the multiplicative value as 43.5.
- Bell Telephone Laboratories, Incorporated (1979). "Unix Seventh Edition Manual". Murray Hill, NJ: Alcatel-Lucent Bell Labs. Retrieved 2011-04-23. Online archive of the manual pages included with the Seventh Edition Unix distribution tapes.
- "Heirloom Project CVS repository". http://heirloom.cvs.sourceforge.net/viewvc/heirloom/heirloom/units/.
- Mariano, Adrian (2014). "Units: A Unit Conversion Program and Scientific Calculator". Boston, MA: Free Software Foundation, Inc. Retrieved 2014-03-29..
- As specified from several NetCDF conventions, e.g.:
- COARDS—"Conventions for the standardization of NetCDF files". 1995-02-10.
- CF—"NetCDF Climate and Forecast (CF) Metadata Conventions". 2014-03-29.
- (German language) « GNU Units 2.0 berechnet Feet und Inches », linux-magazin.de, July 3, 2012.
- Technical Paper No. 410, Flow of Fluids through Valves, Fittings, and Pipe. New York: Crane Co. 1985.