How to Calculate BTUs for an A/C Unit

Calculate the area of the space to be cooled. To determine the area of rectangular spaces, multiply the length of the space by the width of the space. For triangular areas, multiply the length of the space by the width and divide by two. As an example, a rectangular room has a length of 10 feet and a width of 10 feet. Its area is 100 square feet.

Determine the base BTU requirement your air conditioning unit needs by referring to a BTU chart. In the example, a 100-square-foot room will need an air conditioner with at least 5,000 BTUs.

Subtract 10 percent of the BTUs determined in Step 2 if the space faces north or is heavily shaded. In the example, the air conditioner is heavily shaded. Multiplying 5,000 by .10 equals 500. The number of BTUs needed is now at 4,500.

Add 10 percent of the BTUs determined in Step 2 if the space is very sunny. In the example, the space is not sunny, so you wouldn't add any additional BTUs.

Add 15 percent of the BTUs determined in Step 2 if the space is poorly insulated. In the example, the room is poorly insulated. Multiplying 5,000 by .15 is 750. Adding 4,500 and 750 equals 5,250 BTUs.

Add 4,000 BTUs to the total requirement if there is a kitchen in the space. In the example, there is a kitchen in the space, which means the total BTU requirement is now at 9,250.

Add 600 BTUs per person to the total requirement if people regularly occupy the room. In the example, one person regularly occupies the room, which means that the total BTU requirement is 9,850. This space needs an air conditioner with a BTU rating of approximately 9,850.