There are two types of AC units: portable models, which can be anything from window units to free standing types, or whole-house central air units which are routed through existing duct work. All portable air conditioners are rated by British thermal units, but central AC units are rated by the ton. Each ton equates to 12,000 Btu each. You can size both of these types of units by using the square footage of a room or a house.
Measure the width of the room and then multiply it by the length of the room to get the square footage. As an example, a 10-foot by 9-foot room equals 90 square feet.
Apply the square footage to size the correct air conditioner for that room. In general, a 7,500 Btu air conditioner will be sufficient enough to cool down a room that is less than 150 square feet. A 10,000 Btu air conditioner will cool down a room of 300 square feet. Such things as second story rooms, kitchens, and sunny or shaded rooms will require larger or smaller sized air conditioners as the situation warrants. Visit the Air Conditioner Home website and use the table to find the right air conditioner for your square footage for any room in the house.
Add the square footage of all the rooms together to get the square footage of an entire house. You will be able to determine the tonnage and size needed of a central air system to cool off the entire house. A 1 ton rating on central air will cool up to 450 square feet; 900 square feet will require a minimum 2-ton size central air unit, and 1,350 square feet will require a minimum of a 3-ton central air unit.