Draw a sketch of your house on plain white paper. Include all exterior walls.
Start at the corner of one exterior wall and use a measuring tape to measure the length and width of each wall, rounding off the measurements to the nearest tenth of a foot or nearest inch. A tape measure that includes tenths of a foot to display linear footage makes calculations easier. Write down the measurements in linear feet.
Multiply the measurements of each exterior wall. For instance, multiply the measurements of a 30-by-60-foot wall for a total of 180 square feet.
Add the square foot totals for each wall measured to arrive at the total square footage of the house interior. Since you are measuring for air conditioner size, you might need to subtract the square footage of unfinished areas that are not heated or cooled, such as basements, crawlspaces and garages.
Use the method for determining the air conditioner size in BTUs, or British Thermal Units. A ton is the usual measurement unit for the cooling capacity of air conditioners. One ton in air conditioning capacity -- the amount of heat it takes to melt a 1-ton block of ice -- is equal to 12,000 BTUs.
Find the recommended ton of cooling for the square footage of your house in the free guidelines available. One company’s guidelines spread the ton-to-square-footage ranges across five U.S. climate zones to help with sizing. For instance, a 1.5-ton unit is recommended for a range of 600 to 1,100 square feet, with a more precise recommendation of 600 to 900 square feet for the Southern United States. The other recommended ranges are a 2-ton unit for 901 to 1,400 square feet; 2.5 tons for 1,201 to 1,650 square feet; 3 ton, 1,501 to 2,100 square feet; 3.5 tons, 1,801 to 2,300 square feet; 4 tons, 2,101 to 2,700 square feet; and 5 ton, 2,401 to 3,300 square feet.
Convert the recommended unit tons to the corresponding BTU number. For instance, 36,000 BTUs is the cooling capacity for a 3-ton air conditioning unit.