How to Calculate the Heat Load for an Air Conditioner

Determine the size of the room or rooms you plan to cool with the air conditioner. Use your tape measure to record the length and width of the air-conditioned space. Multiply the two measurements to find the area of the space. For example, if your room is 10 feet by 10 feet, the area to be air conditioned is 100 square feet.

Divide oddly shaped rooms into smaller, more easily measurable parts. For example, if you have a room with one end that is triangular in shape, use your tape measure to record the length and width of the rectangular portion of the room, and measure the triangular portion separately. Measure the length and width of the triangle, then multiply the two numbers and divide the product by 2. Add the area of the triangular area to the area of the rectangular area, and you have the area of the entire room.

Calculate what size air conditioner your space will require based on the square footage you have measured. For rooms of 150 square feet or less, a 5,000 BTU air conditioner will be suitable. Add 1,000 to 2,000 BTU for every additional 100 square feet of space you plan to cool. For example, 500-square-foot spaces require about 10,000 BTU, while 1,000 square feet require about 20,000 BTU. (Use an online air-conditioning calculator to simplify the process.)

Factor in any other variables. If you are installing an air conditioner in your kitchen, you'll need an extra 2,000 to 4,000 BTU of necessary cooling power to make up for the heat generated by your oven and other appliances. If there are more than two people regularly present in the air conditioned room, the body heat must be factored in. In rooms which have regular sun exposure, or in particularly warm climates, an increase off at least 10 percent in BTU rating may be necessary. The same is true of spaces with little or no insulation as the cool air continuously leaks out. On the other hand, you may reduce the BTU rating by 10 percent for interior spaces that remain shaded or are located in cooler climates.