Tinytree,
you cannot perform calculations in an access table. They are strictly for storing data.
If you wish to create a set of rows and columns that include both data, and calculations, then what you need is a query.
So, let's say you have a table called tblMyTable that contains a field A1, and a field A2.
You would create a query that sources its data from tblMyTable, and returns A1, and A2, and also has a third calculated field that we'll call A3. A3 contains the calculation you mentioned.
So, the SQL for your query might look something like:
Code:
Select A1, A2, Nz([A1],0) + Nz([A2],0) As A3
From tblMyTable;