Measure the square footage of each room that is attached or will be attached to the AC unit. Multiply the length of the room by the width of the room. Remember to calculate all rooms including hallways. You do not need to measure closets or other rooms that are not connected and are sealed off from the AC unit. Measure connected rooms, such as an openfloor plan kitchen and living room, separately.
How to Figure the Size of an Air Conditioner
Sizing an AC (Air Conditioner) unit requires calculating square footage of a room or house. Replacing a damaged unit makes the job easier. The model number on the outside fan has a two digit code that corresponds with the size of the unit, measured in tons. Take the model number to any dealer and they can determine the unit's size from the code. You can also calculate the square footage of the area connected to the unit, convert the area to BTUs, and convert the BTUs to tons.
Instructions


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Convert the square footage into total BTUs. One hundred to 150squarefoot rooms (most bedrooms) are 5,000 BTUs. Rooms from 150 to 250 square feet require 6,000 BTUs and rooms from 250 to 350 square feet need 7,000 BTUs. Add 4,000 BTUs for the kitchen, and 1,000 for bathrooms.


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Add each room's BTUs together to determine the total house BTUs.

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Convert the total BTUs to tons, which is the standard measurement of AC units. 1.5 tons equals 18,000 BTUs, 2 tons is 24,000 BTUs, 2.5 tons is 30,000 BTUs, 3 tons is 36,000 BTUs, 3.5 tons is 42,000 BTUs, 4 tons equals 48,000 BTUs, and 5 tons equals 60,000 BTUs. The ton size is the size of AC unit needed to properly cool your house.

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References
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