As a follow-up to my previous reply, note that newer versions of Access allow you to do calculated fields in Tables, but it is highly advised NOT to do this, and to do it in queries instead.
Here is an article which discusses that:
Why One Should Use Queries instead of Calculated Fields for Access Tables.
Another reason not to is that it violates the Rules of Normalization, and other database programs like SQL do not allow this, so if you ever need to convert the database or interact with the database programs, you will probably have issues.
Simple rule of thumb to follow:
Never store in a table that which can easily be calculated in a query.
There is seldom ever a need to. You can use a Query for almost anything that you use a Table for.