I'm not sure if it's open source, but there's a command-line tool that came with my Mac that may also be on your machine to do unit conversions. It's called "un

I'm not sure if it's open source, but there's a command-line tool that came with my Mac that may also be on your machine to do unit conversions. It's called "units". Even if you don't want to call it directly, you may be able to use the conversion table files that are stored somewhere for it to use. As for calculations, if they are basic, interpreting the calculation as Ruby or Python code might work, though it risks the user entering something more dangerous that might need to be checked for.