Justin,

Do you mean a protect form using formfields?

Here is an example.  Open a new blank document. View>Tool
Bars>Forms

Insert 3 formfields
Double click the first formfield and select Number as the
type and check calculate on exit
Do the same with the second formfield
Double click the third formfield and select caculation as
the type.  In the expression field enter =Text1+Text2
Protect the form and give it a try ;-)

The variables Text1 and Text2 are the bookmark names of
the first two text formfields in the document.  You can
change these by double clicking the field and entering
your custom bookmark name.

If you are not using a protected form you will need to
somehow identify the reference location of the variables.  
You can do this (but not recommended) as a bookmark in
free text.  Another way is a table cell reference.

Say you bookmark two variables a and b.  You then insert a
formula field {=({a}+{b})} where  { } pairs are entered
with CTRL+F9.  Unlike the formfield method above, these
type fields will not update automatically.  You can right
click the field and manually update individual fields,
CTRL+a and then F9 to update all fields, or it Update
fields is selected in print options you can update at
print or toggle in and out of print preview to update
fields.

The table methd uses cell references.  Enter your varibles
in cell A1 and B1 (thats row 1, column A and B).  In C1,
enter the field code {=(a1+b1)}.

Type =formula in Word help for more on using forumla
fields.
There is

Hi Justin,

To see how you can use formulae in Word, download the document at:
http://www.wopr.com/cgi-bin/w3t/showflat.pl?Cat=&Board=wrd&Number=365442
(url all one line)

Cheers