GUI calculators may be user-friendly, but they don't offer much comfort for a command-line power user. Fortunately, console-based tools such as bc, Genius, and Calc offer distinct advantages over their GUI counterparts.
Command-line calculators offer most if not all of the features GUI calculators have, and have a lot more going for them:
While Genius is the most advanced (and probably stable) of the aforementioned console calculators, Calc is the most user-friendly, and provides more capabilities than the average user will need. You can find Calc in the repositories of many distributions, and it can also run under Windows.