In the Formula box check the text between the parentheses to make sure Word includes the cells you want to sum and click OK. Place the cursor at the blank cell of the first row then click layout Formula see screenshot. The first row is Row 1 the second row is Row 2 etc.
Click in a cell that should contain the sum of the rows.
Go to Insert tab. Then find Ink Equation button at the left side of the Equation Tools. In this article Im going to talk about how you can use formulas inside tables in Word. On the Layout tab under Table Tools click Formula.