Raul, if you want just a normal calculated field, you can do it with a query. That's easy. We will cover that in Access 220. I also have a tutorial on it in the Tips & Tricks section.

Now, if you want that value to calculate AND be stored in a table field (perhaps so you can edit it), you have to do some more work. You need to use some VBA programming (or a macro - but I prefer the VBA). I cover it in Access 303 because it's a little more advanced, but again I have a Free Tutorial explaining how to do it.

There is a LOT more to it than I cover in the Free Tutorial, of course. Get the full Access 303 class for complete details.