Calculations

A text form field can also be used to perform a calculation in which the result is based on the value in other form fields.
Calculation - Tells Word that you want the field to perform a calculation.

You must choose Calculation as the type of data.

All formulas begin with an equal sign
To specify values in a table you can use cell references where "A" is the first column in the table and "1" is the first row in the table.
=SUM(ABOVE)
=SUM(A1,A2)
=Bookmark1 + Bookmark2 'since every form field has a bookmark associated with it
Do not check the "Calculate on Exit" option