If you have a google search bar, you can use it to perform calculations, and if you include the = sign, it gives you the answer in its little auto-complete suggestion thingy without you even having to actually go to google.
If you're on a Mac hit command-space to bring up Spotlight, and then punch in your calculation. Spotlight will display the result (don't use commas). Most people playing are probably playing in a window since Glitch is a relatively small pixel-depth. I know I could probably fit 4 copies of glitch on my screen at the same time, plenty of space to bring up a calculator. Personally though, I use my TI-89.
Also, use the google spreadsheets people have made, they're very useful for this sort of thing.