Actually, google will convert for you. No need for a separate program . Searching on "55km to mi" gives a first hit of 34.1754156 mi, then a more normal set of hits to other sites. You can do the same thing with quite a lot of other units. Very handy if your brain is programmed in English system. Or in metric.

My brain does both, but it kind of half-asses it. Some things I can only work in metric. Others I can only work in English. And some I never know quite what units my brain will use.