Calculate the square footage in the area the air conditioner will cool by multiplying the footage. For instance, if a room measures 10 feet wide by 12 feet long the square footage is 120 square feet, or 10 times 12, and a larger space such as 20 feet long and 22 feet wide is 440 square feet, or 20 times 22.
Determine the location of the space. Consider issues like whether the room is located on the south side of the house, where it receives more sunlight, or on the north side, where little sun reaches the room. The more sunlight that shines through windows, the more Btu the room requires to keep it cool. A kitchen requires higher Btu than a bedroom that is shaded from the sun because of the heat from cooking.
Estimate the number of Btu needed for the room size. For a room size of 150 to 250 square feet, you need a 7000 Btu capacity, while a large space of 1,000 to 1,200 square feet requires 21,000 Btu. Take into consideration the location of the room and subtract a shaded and well-insulated room by 10 percent Btu. By the same token, a room such as a kitchen, or a space with all day sunlight filtering through would add 10 percent Btu.