GNU Units can convert quantities expressed in one unit to an equivalent in a
difference scale.  Compared to traditional BSD implementations, it offers more
units, non-linear conversions (e.g. between degrees Celsius and Fahrenheit), a
more lenient input parser, and tab completion.  It can also be used to perform
calculations taking physical units into account, and provide the definitions of
units (e.g. "joule = N m = 1 kg m^2 / s^2").

Homepage:
https://www.gnu.org/software/units/
