If you like to use Windows Calculator to perform quick calculations while in Excel, you can save time by adding it to your Quick Access Toolbar. This should work for any version of Excel that has the Ribbon.

Click Customize Quick Access Toolbar and then choose "More Commands".

On the left hand side choose "Commands Not in the Ribbon" from the drop down menu, and you'll see Calculator in the list on the left pane. Just click the Add button to add it to the toolbar.

You now have the Calculator icon ready to use in the toolbar.

Some of the newer Microsoft keyboards also have a button that launches the calculator for even quicker access.