When I’m sitting at my Mac and need to do a quick bit of calculation, I typically launch the Calculator app with my app launcher of choice, Alfred.
If you don’t want to launch the app, click on the numbers, or enter in the calculations via that graphical interface, you can just use Spotlight.
First off, activate Spotlight by hitting the Command and Space keys on your keyboard, or by clicking on the little magnifying glass in the upper right of your Mac’s screen.