Insert a formula in a Word table cell

To insert a formula in a table cell:

Choose Table → Formula.
Enter a formula in the Formula box.
You can select a function from the Paste function list to add it to the Formula box.

Word formulas are similar to Excel's, although Word also includes a keyword, ABOVE, that you can use to refer to cells above the cell where the formula has been created.
Select a Number format.
Click OK.

Word creates a field in the cell. The field contains the formula.

Fields do not update automatically. Select the field and press F9 to update the field.