This article is based on legacy software.
Word can do basic calculations within a table, rather than forcing you to perform the calculations by hand. If your table contains several calculations, however, a spreadsheet created in a program like Excel may be a more efficient option.
The same principles that are used to do calculations in Word are used in Excel worksheets. Instead of entering the actual value you want to use for the calculation, you will be referring to the cell containing the value. The cell reference is in the form of "Column ID, Row ID." The columns are referred to by letters starting at A. The rows are referred to by numbers starting at 1. The first cell of the table (i.e., first column, first row) is referred to as A1.
This document explains how to use calculations within tables.
Formula Examples
You need to designate the appropriate actions when writing a formula, much like pressing addition or multiplication keys on a calculator. These actions are referred to as operators. The following comprise the basic formula operators:
Addition  +  Multiplication  * 
Subtraction    Division  / 
The following table is an example of a completed travel budget, created using formulas in a Word table, that could be included in a proposal for attending a conference. Following the first table is a description of the formulas used to perform the calculations within the table (indicated by the gray shading).
Formula for  Actual Formula  About the Formula 

Hotel  =69.95*3  Computes the total cost for the hotel stay by multiplying 69.95 by 3. 
Meals  =50*4  Computes the total cost of the meals by multiplying 50 by 4. 
Total Conference Budget 
=sum (above)  Calculates the total of the costs by adding the values above the formula (B2 through B6). 
Department Contribution 
=b6b7  Calculates the department contribution by subtracting the grant request from the total conference budget. 
Inserting Formulas
To insert a formula, determine the values or cell references required for the formula and then follow these instructions:

Place your insertion point in the cell where you want to place the formula.

From the Layout tab, in the Data group, click Formula.
The Formula dialog box appears.
HINT: Similar to Excel, based on the numbers in the table and the location of the cell in which you want to place the formula, Word will guess what type of formula you may want (e.g., to add all cells to the left of the formula, =SUM (LEFT) may be placed in the Formula text box).

In the Formula text box, type the desired formula.

If necessary, from the Number format pulldown list, select the desired format for the result.

Click OK.
The formula is inserted.
Recalculating Formulas
To update values in a table, recalculate the formula(s) using one of the following methods.
Recalculate the Value of an Individual Cell: Keyboard Option

Place your insertion point in the cell, before the numerals.

Press [F9].
OR
Press [Alt] + [Shift] + [U].
The formula is recalculated.
Recalculate the Value of an Individual Cell: Mouse Option

Place your insertion point in the cell, before the numerals.

Right click the cell » select Update Field.
The formula is recalculated.
Recalculating the Values of the Entire Table

Place your insertion point within the table.

From the Layout tab, in the Table group, click Select » select Select Table.
The entire table is selected. 
Press [F9].
OR
Press [ALT] + [Shift] + [U].
All formulas are recalculated.