How to Calculate the AC for a Home

Measure the area of each room and hallway in your home that will need to be air conditioned. For rectangular and square rooms or hallways, multiply the room or hallway's length by its width to obtain its area. For triangular rooms or hallways, multiply the room or hallway's length by its width and divide by 2 to obtain its area.

Add the room and hallway areas together to calculate the total square footage of your home that requires air conditioning. As an example, a home has a square footage of 1,000 square feet that will be requiring air conditioning.

Consult a BTU table to determine the base BTUs needed for your home. Using the example, a house with a 1,000 square foot cooling area requirement needs an air conditioner with at least 18,000 BTUs.

Add to the base BTUs you require 4,000 BTUs for any room in your home that is below an area that is not insulated, but requires air conditioning. In the example, the home has a room under a garage that is not insulated; therefore its BTU requirement is now up to 22,000.

Add 4,000 BTUs to the requirement calculated in Step 4 if you have a kitchen and it will be cooled. In the example, the home has one kitchen that needs to be cooled; therefore the home's BTU requirement is now at 26,000.

Add 1,500 BTUs to the requirement calculated in Step 5 for every window that receives sunshine for the majority of daylight hours. In the example, the home has 3 windows that receive 7 hours of daily sunlight; therefore the home's BTU requirement is 30,500.