Qalculate! is my favourite GTK and Qt-based calculator that's great at accepting all sorts of inputs, even solving for X: https://github.com/Qalculate/qalculate-qt
… but most importantly it can calculate with units and show the result converted to your liking e.g. you can write
10 Nm×2pi×20/minute to W and Qalculate will show the result as 209,4395102 W. When you get used to that, you don't want to use any calculator that doesn't have that feature any more.
e.g.
> 100 GB/minute to kB/s