Measure the interior length of your home. For intervening walls, pick up the measure on the other side of the wall. Add a few inches to the final measure for each wall.
Measure the interior width of your home. Use the same method.
Multiply the length and width. For instance, if the home is 26 feet by 37 feet, the result is 962.
Repeat the measuring for each floor. If your home has the same profile for each floor, and they are the same size, multiply the square footage of your original measure by the number of floors. For example, if the home is two floors with a 962 square footage per floor, the total is 1,962.
View the map on the reference "AC4Life: How to Size Your System." Find your area and note the zone. For instance, if you live in Missouri, you are in Zone 3.
Multiply your total square footage by 35 for Zone 1, 40 for Zone 2, 45 for Zone 3, 50 for Zone 4 or 60 for Zone 5. In this example, the 1,962 square footage times 45 (for Zone 3) is a 88,290 BTU requirement for the cooling system.