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Autosum selected cells

Windows shortcut 
Alt=
Mac shortcut 
T

If you select just the cells you'd like to sum, SUM formula(s) are placed below the selection. If you select empty cells to the right and Excel will place SUM to the right and SUM horizontally. If you select empty cells below and to the right, Excel will add SUM formulas to cells below and to the right.

Related videos

The videos below demonstrate this shortcut.
In this lesson, we cover shortcuts you can use when working with Excel's functions.
In this video, we compare formulas and pivot tables side-by-side to build the same reports to demonstrate how powerful and easy to use pivot tables are.
In this video, we'll take a look at a few reasons why you shouldn't hardcode values that may change into your formulas.