It is a bit slow but has decent features, including some physical units support.

I set up Tilda (or another Guake equivalent; I tried a bunch of them and can’t remember on which I settled in the end) to run it automatically when a terminal is opened, when pressing Scroll Lock. It’s quite neat: when I need to do a quick calculation, I just hit Scroll Lock, which spawns a drop down terminal with the focus; then type whatever I want to calculate; then control-D to dismiss the drop down terminal and I am back in whatever I was doing without moving either hand off the keyboard.

I do more or less the same thing on my Mac with Alfred.