There’s no need to break-out the calculator (either real or software) when you want to do a quick calculation. Just use Spotlight, as follows.
The Spotlight search tool also includes a simple calculator. Simply activate Spotlight search by clicking the icon at the top right of the screen or by pressing Command + Space and typing the math expression (for example, 2+2 or 43-22). You’ll see the response immediately underneath in the form of a search result. For multiplication and division, use * and /, respectively.
Constants such as pi are understood (just type pi), and for square roots type sqrt followed by the number in parentheses, such as sqrt(9). cos, sin, and tan also work as you’d expect (for example, type cos(36)).
By hovering your mouse over the result so that it’s highlighted, you can copy the math into the clipboard using Command + C and then paste it into any application with Command + V (or by clicking Edit->Paste on the application’s menu). You can also drag and drop the result from the Spotlight list to a document to paste it in.
Spotlight’s math function is actually an extension of the math abilities built into the command line of OS X. To learn more about what commands you can use, open a Terminal window (Finder->Applications->Utilities->Terminal) and type
man math and then look under the List of Functions heading (scroll up and down in the Terminal window using the Up/Down cursor keys; you can search by hitting the forward slash, hitting the slash followed by Return to repeat the search).