Writing Formulas
There are several different types of calculations that can be performed in a Word table. Some of the most commonly used functions are:
AVERAGE: Use this to calculate the average of a list of numbers.
COUNT: Use this function to display the number of items in a list.
MIN: This function is used to display the smallest number in a list.
MAX: This function is used to display the largest number in a list.
PRODUCT: Use this function to multiply values.
SUM: Use this function to add values.
However, for some formulas, the function names can be omitted. For instance, you can use mathematical symbols instead, such as the symbols to add (+), subtract (-), multiply (*), or divide (/). Click on the following image for examples of various ways formulas can be written:

To Insert a Formula
1. Place your insertion point in the cell where you want the total to be.
2. Click Table on the menu bar and choose Formula. (Sometimes Word will try to “guess” what formula you need. If it guesses wrong, just delete the formula – but keep the equal sign.)
3. Choose your function from the Paste Function drop-down list followed by your formula in parentheses, -or- omit the function name and just type in your formula.
TIP: Instead of typing in the cell names, you can use "left" or "above" in your formula. For example, if you were placing your total at the bottom of a column, you could write your formulas like this: =SUM(above). Or if you were placing your total to the right of a row of numbers, you could write it like this: =SUM(left).
Calculating With Bookmarks
Suppose you want to insert an amount from your table into the body of your document. Or, maybe you want to calculate values from two different tables? These things can easily be done by creating bookmarks.
In the following example, we will add totals from two different tables and insert the calculated total in the body of the document:
Step 1: Create the Bookmark
1. Select the value in your document that you want to have bookmarked (the value does not have to be within a table).
2. Click Insert on the menu bar and choose Bookmark.
3. Name the bookmark. (NOTE: You cannot have any spaces in your bookmark name, and the name cannot begin with a number.)
Give your bookmark a unique name - there cannot be another bookmark within the same document with the same name. If there are any other bookmarks in your document, you will see them listed.
Bookmarks, continued...
Step 2: Insert the Formula
1. Place your insertion point at the location where you want the total to be displayed. (It can be in an empty cell within your table -or- anywhere in the body or your document outside of a table.)
2. Click Table on the menu bar and choose Formula. (You can still use the "Table - Formula" function even when inserting a formula outside of a table.)
3. The Formula dialog box opens. Enter your formula in the Formula Field. To insert the bookmark, select the bookmark(s) from the Paste Bookmark drop-down list. All bookmarks created in your document will be listed.
4. Choose a Number Format if applicable, then click OK when your formula is complete. 
Conclusion
One last final note: when you're calculating a column or row of numbers, make sure there is a numerical value in each cell. If you leave a cell blank, the calculation may stop at that point. This is typical when using "above" or "left" in your formula. So rather than leaving a cell empty, put a zero there instead.
Now that you have all the skills you need to perform calculations in Word - go show off your skills!
Word 2003 Tables
Everything you need to know to create tables in Microsoft Word.