xen-calc is a multi-purpose calculator/converter for microtonal (AKA: xenharmonic) music theory. It supports conversion between cents, just ratios, and EDO, FJS, and color notations. It can also calculate interval multiplication/addition, best rational and EDO approximations, and much more. Check out the about page for a full list of features.

Don't know where to start? The about page has tons of click-able examples (like this: 11/9) as well as some written-out examples of using this tool.