GNOME supports unit conversion too, by pressing Super and typing in the queries. And iOS too. Not sure about Android though, because IIRC the search bar just directs you to Google search.

Consider using the "units" program. It's excellent and can handle almost any conversion and unit-full math expression.

Can also do them on iOS by swiping down the home screen

KDE will do it in Krunner. All units and currency

Krunner is super finicky about it though, especially since it won't handle even the simplest of mixed units (eg. neither 6ft 2in or 6'2" works, where 6ft, 6', 2", and 2in all give conversions on their own).

...and then after it fails, if you hit enter or click at the top of the results like you would to copy the value, it launches whatever random program burbled to the top when it gave up.