Measure the square footage of all the rooms in the home that will be cooled, and then compare that number to the government's recommended British thermal unit capacity list. A 2,500-square-foot house requires 34,000 Btu.
To measure a square or rectangle shaped room, measure the length of the room as well as the width, and then multiply those numbers together to get the square footage. Do this for each room, and add them together to get the total home square footage.
For an odd-shaped room, try to break the room down into a rectangle and triangles. For a triangle measurement, measure the length and the width, and multiply them, and then divide by 2.
Other factors in figuring the Btu capacity include the type of room. If the room is naturally dark, reduce the size by 10 percent, and if it is naturally sunny, increase the size by 10 percent. If the room is the kitchen, add on 4,000 Btu to the recommended amount. So a 1,500-square-foot house would require 24,000 Btu, but a 1,500-square-foot house with a kitchen requires 28,000 Btu.
The air conditioner not only cools the air, but it also reduces the humidity in the house. If the size of the air conditioner doesn't match the house, it will not reduce the humidity as efficiently.