Here are two quick ways to do a sum in Excel:
Excel’s AutoSum automatically inserts a SUM formula. Just type Alt+= (‘Alt’ and ‘=’ simultaneously) under (or to the right of) the numbers you want to sum.
Excel will guess what range you want to sum. It usually guesses the range correctly. If not, you can adjust it manually by resizing the colored box (just drag one of the corners). See the image below:
You can just manually edit the sum range to be anything you want. So even if Excel guesses incorrectly, at least you have a good shell of a formula.
There’s an even faster way, though, if you don’t need a permanent formula.