I like the Google Calculator, but don't use Google services and the Google Calculator can't work without Play Services for some reason. So I've tried all the popular open source ones and found https://github.com/sadellie/unitto to be the closest in experience (material design) and accuracy (the tan(355/226) test works). It also has conversions for other units, and even currency using https://github.com/fawazahmed0/exchange-api that is accurate enough for a rough estimate if you let it have Internet access.