Look up the number of square feet you expect your central AC unit to cool by checking your home's blueprints. If you cannot find the blueprints and do not know the square footage, you can calculate it by measuring the dimensions of each room and hallway, calculating the areas and then adding up the total.
Use a sizing chart (see resources) to find the base number of Btu you need for your home. For example, if your house is 2,200 square feet, you would need about 28,500 Btu.
Count the number of well-shaded rooms and multiply the result by 1,000 Btu. Then subtract the result from the base Btu in step 2.
Count the number of rooms that are on the sunny side of the home and get significant sunlight and multiply that number by 1,500 Btu. Add this result to the base Btu from step 2.
Modify the estimate in step 2 if your home is not fully insulated. Each room that is not insulated adds 4,000 Btu to the base Btu amount from step 2.
Add 4,000 Btu to the base Btu total for each kitchen you have in your home. The additional heat created in kitchens results in a higher cooling demand.
Determine if any of the rooms in the home will have more than two people occupying it on a regular basis. If so, add 600 Btu for each person beyond the second that will be in the room. For example, if three people will share a bedroom, you would add 600 Btu.
Things You Will Need
- Blueprints or measuring tape
- Some air conditioners list their capacity in tons rather than Btu. One ton equals 12,000 Btu.