GNU Units — The Complete Guide: Unit Conversion and Unit-Aware Calculation from the Command Line
GNU Units is a long-standing command-line tool from the GNU Project, under GPL-3.0 or later, free for any use. You type what you have and what you want, and it converts — or tells you at once that the dimensions do not match.
GNU Units
GNU Project (Adrian Mariano and contributors)
Command-line unit converter and calculator that tracks units through a calculation, with a database of thousands of units.
Get it from the vendorWhat you get
- Thousands of units — a broad database including historical, scientific and engineering units.
- Non-linear conversions — temperature and other units that are not a simple factor.
- Sums of units — such as feet and inches together, for input and output.
- Unit-aware calculator — evaluate a whole expression while units are carried along.
- Extendable units file — add your own definitions.
How it works in practice
Type an expression like torque times angle and ask for the result in joules, or compute flow from velocity and cross-section area and get litres per second at once. If there is a dimensional mistake, it refuses to convert and shows what it got, which makes a great sanity check for hand calculations. It can be called from scripts. Limitation: there is no graphical interface, and on Windows you use a binary build maintained by a volunteer.
Related tools
For a graphical unit converter with free-form combinations — ConvertAll. For a desktop calculator with units and a friendly interface — Qalculate!. For a documented engineering calculation sheet — SMath Studio.
ConvertAll
Doug Bell
Flexible unit converter that lets you combine units freely, with SI prefixes, fractions and results in engineering notation.
Get it from the vendor
Qalculate!
Hanna Knutsson
A smart desktop calculator — full unit conversion, physical constants, uncertainty propagation and symbolic algebra.
Get it from the vendorThe bottom line
A small, powerful tool for engineers who live in the terminal: precise conversions and unit-aware calculation that catches mistakes before they reach the report.