How to Calculate AC Requirements

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Calculate the requirements for cooling a home before purchasing an air conditioner.

In order to purchase the right air conditioner (AC) for your home, you must first know what size air conditioner is needed to cool the home. Calculating the AC requirements for any particular room may seem like a daunting task, but the formulas required to calculate British thermal units (BTU) requirements are easy to use. There are also many on-line calculators that provide a more accurate figure and take into account more variables, such as wall materials, orientation and geographical location.


Step 1

Calculate the square footage of the area you need to cool. Measure the length and width of the room and multiply the number together. The result is the square footage.

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Step 2

Multiply the square footage by 25.


Step 3

Add 400 to the previous result for each person that will occupy the room. For example, if you are cooling a bedroom that is shared by two people, then you would add 800.

Step 4

Increase that number by 1,000 for each window in the room. The resulting answer will give you an approximate amount of BTUs that you will need to cool the room. So a room with two windows, two people and 250 square feet, under these calculations would need 9,059 BTUs to cool the room.


When purchasing an air conditioner, make sure that the unit can handle more than the BTUs required to cool your area.

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