It's very easy to use: just select the input unit and output unit.
A notification will appear in the status bar if the units selected aren't compatible.
Also, ConvertAll allows you to add new units of measure to its database and carry out basic operations with them.
It includes a search engine that makes finding units easier. You can search by name or by unit type.
Charge, conductivity, time, data, density, mass, luminescence, pressure, resistance, radioactivity... You'll be able to convert these measurement units and many others with ConvertAll.