Tables in Word 2013 look similar to cells in an Excel spreadsheet, but unlike Excel, you can't start a formula simply by typing an equals sign. Word's formulas can also use a different notation, referring to sets of numbers by their relative position. To add table cells using the SUM function, insert a formula from the Layout tab.
Sum Cells in a Table
Step 1: Start a Formula
Click the target cell in your table, open the Layout tab and click Formula in the Data section.
You won't see the Layout tab until you click a cell in the table.
Step 2: Sum an Entire Direction
Write a formula using the function =SUM to add cells. When you first open the formula window, Word defaults to the formula =SUM(ABOVE). If you want to add all cells above the selected cell, you don't need to change anything. To add cells from a different direction, change the formula to =SUM(BELOW), =SUM(LEFT) or =SUM(RIGHT).
To add cells from two directions, write both separated by a comma, such as =SUM(ABOVE,BELOW).
Step 3: Change the Number Format
Choose a number format, if needed. Without a number format selected, the formula's result appears without any formatting, akin to Excel's "General" formatting. Most of the time, you don't need to use a format. Word also lets you write your own format using the same system as in Excel. Click OK to finish the formula.
Step 4: Sum Specific Cells
Refer to cell locations in a SUM function to add specific cells rather than an entire direction. Even though Word doesn't display row and column headings like Excel, it uses the same terminology: rows use numbers and columns use letters. For example, to add the cell three to the right, three down, and the cell below it, use the formula =SUM(C3:C4).
The Paste Function menu provides a shortcut for adding new functions to the formula. Because Word inserts the SUM function as soon as you open the window, you don't need to use it for this task.
Insert an Excel Spreadsheet
Another way to add cells in a Word document is to insert an Excel spreadsheet. Click Table on the Insert tab, as if inserting a table, but instead of choosing a table size, click Excel Spreadsheet. Inserted spreadsheets work exactly like spreadsheets in Excel, allowing you to simply type the function =SUM in the desired cell.
Inserted spreadsheets support all of Excel's functions, whereas Word has a much smaller selection of functions.