There are many ways to do calculations from the command line. Here are a couple that are worthwhile.
Note: See the parent document: Using the Terminal as a Daily Driver for more about terminal productivity applications.
Qalc
Qalc is the command line version of Qalculate. I was able to install just the command line version through my distributions package manager.
It supports an interactive mode as well as a simple command line mode. Nice thing about these kinds of calculators, they can be used to do calculations in a script.
bc
I believe this one was already installed with my distro. Fewer features than qalc, but should meet most needs.