KCalc

KCalc is the default calculator for the KDE desktop environment. Even though in itself not mobile-friendly, you can apply some quirks to make it somewhat usable.

Scaling in Phosh
Use Phosh's scaling command so that KCalc fits on your mobile screen. Be aware that KCalc's App-ID is :

Larger Buttons
Go to. In the section you may tweak your button & display font sizes.