GNU Units is a powerful tool that allows you to convert quantities between different systems of measurement. It can handle various types of conversions, including nonlinear conversions like Fahrenheit to Celsius and wire gauge. In addition to unit conversions, GNU Units can also be used as a scientific calculator that tracks units in its calculations. You can create complex mathematical expressions involving different units and ensure accuracy and dimensional consistency. The program comes with an extensive data file, but you can also provide your own data file. GNU Units is available for download on the GNU website and is free software released under the GNU General Public License.
https://www.gnu.org/software/units/