How to Measure for the Correct Size Air Conditioner

All air conditioners are measured by the amount of their cooling capacity, or BTUs (British Thermal Units). Central air conditioners are rated by the ton, with each ton equal to 12,000 BTUs. Conversely, room or portable air conditioners are measured just by the BTU rating of the unit. You can measure for the correct-sized air conditioner for use in either either a specific room or a whole house by doing simple mathematical calculations.

Things You'll Need

  • Tape measure


    • 1

      Measure the square footage of a room with a tape measure. Square footage is obtained by multiplying the length times the width. As an example, a 12-foot-by-12-foot room is 144 square feet.

    • 2

      Determine the correct air conditioner needed to cool off the area measured. An air conditioner rated between 7,000 and 10,000 BTUs will cool a 100- to 200-square-foot room. An air conditioner rated between 9,000 and 12,000 BTUs will cool a 250- to 350-square-foot room. An air conditioner rated between 12,000 and 16,000 BTUs will cool a 350- to 450-square-foot room.

    • 3

      Reduce or increase your BTU calculation if special circumstances apply. If the room is shaded by trees or buildings, reduce the amount of calculated BTUs by 10 percent. If the room is on the sunny side of the building or on the second floor, increase your BTU calculations by 10 percent.

    • 4

      Add 600 BTUs for any room calculation if there will commonly be more than two people in the room at the same time.

    • 5

      Increase the BTU measurement by 4,000 BTUs if the air conditioner will be located in a kitchen.

    • 6

      Add the measurements of all the rooms sizes together. Allow for kitchens and second-story rooms, then calculate how many tons you will need on a central air conditioning unit for whole-house cooling.

Tips & Warnings

  • The higher the Energy Efficiency Rating of an air conditioner, the more efficient it will be, and the less it will cost to run over extended periods of time.
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  • Photo Credit Jupiterimages/ Images

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