Step 2 Now click the Layout tab and then click the Formula button. Just switch to Insert tab and click Equation Insert New Equation to toggle on Equation Tools. Use the Formula dialog box to create your formula.
Write a formula using the function SUM to add cells.
In the Formula box check the text between the parentheses to make sure Word includes the cells you want to sum and click OK. If you want to display the sum as a percentage or as currency then click the Number format drop-down menu and select the appropriate format. Typing LEFT to calculate all the data on the left of the blank cell. Click the Layout tab.