certain dealers provide a sizing worksheet to calculate the air conditioning capacity (tonnage) required. The Trane system installed in my house has calculation worksheets with their literature. ask a dealer in your area. they should be able to provide you with one. also, joe is not being silly. there are many detailed calculations that go into the cooling estimate. also, you may not know all of the variables such as insulating envelope of the structure. call local professional dealer and give them your specifics to see what their calculations determine. lastly, i say when in doubt go a little bigger with the unit. better to have excess capacity than not enough.