GNU Units

GNU Units is a long-standing command-line tool that converts quantities between measurement systems, including non-linear conversions such as temperature and sums of units such as feet and inches. It also works as a scientific calculator that tracks units through the whole calculation, so you can type an expression like torque times angle and get energy in the units you want, or spot a dimensional error at once. The units file can be extended with your own definitions. It suits engineers who work in a terminal or want unit conversion inside scripts. Limitation: there is no graphical interface, and on Windows you use a binary build maintained by a volunteer.