How to Choose an Air Conditioner by BTU

Central air conditioners are rated by the ton of cooling capacity, while window units or portable air conditioners are rated by British Thermal Units or BTUs. Each room will have its own BTU requirement, based on the area of the room and the type of room it is. A kitchen with the same area as a bedroom, for example, will need a larger BTU air conditioner to achieve the same cooling effect.

Window air conditioners are rated in BTUs of cooling capacity.
  1. Measure the area in square feet of the room the air conditioner will cool. Multiply the length of the room by the width to get the area. As an example, a 12-foot by 10-foot room is 120 square feet.

  2. Determine the BTUs needed for the measured area. A 100- to 200-square-foot room will require an air conditioner in the 7,000 to 10,000 BTU range. A 250- to 350-square-foot room will require an air conditioner in the 9,000 to 12,000 BTU range. A 350- to 450-square-foot room will require an air conditioner in the 12,000 to 16,000 BTU range.

  3. Reduce your BTU calculation by 10 percent if the room is shaded by trees or buildings. Increase the BTU calculation by 10 percent if the room is always sunny or is on the second floor.

  4. Increase your BTU calculation by 600 BTU if more than two people usually will be in the room at the same time.

  5. Add 4000 BTU to any air conditioner that is going to placed in the kitchen.

  6. Running an air conditioner for any length of time means using more power. Select a high Energy Efficiency Rating (EER) to reduce power consumption.

About the Author

Dale Yalanovsky has been writing professionally since 1978. He has been published in "Woman's Day," "New Home Journal" and on many do-it-yourself websites. He specializes in do-it-yourself projects, household and auto maintenance and property management. Yalanovsky also writes a bimonthly column that provides home improvement advice.