[mrkrabo]

There probably is a replacement in the store already, but being frank, I hate all store apps. The only store app I use is the calculator, because I'm forced to (the classic calculator was removed), and I hate that it takes a few seconds to start up. We're going backwards.

[wfunction]

I never use the calculator except on a brand new installation where I have to compute sector offsets or something.

Use the PowerToy calculator from Windows XP. You can either run it in compatibility mode or patch it so that it doesn't check the Windows version (exercise left for the reader).

Alternatively, use a third-party program like SpeedCrunch.

[g00n]

I tend to use Google to calculate, if posed correctly it will even infer formulas and give you the result. But when I needed math done for me I was also waist deep in Excel all day long and could figure anything I needed.