Fractions to decimal is fairly straight forward for normal US measurements and I remember a few decimal inch to mm numbers but for anything more than that and for most things other than length measurements I use an Android app called Engineering Converter Plus . It has an incredible number of measurement types and units.

https://play.google.com/store/apps/details?id=net.thermofluids.converterplus&hl=en

Works very well and doesn't rely on network connections. I find web sites to be slow at times so I use apps where I can on mobile devices.