A British thermal unit (BTU) is the measurement of how much cool air an air conditioner must produce to adequately cool a given space. Every air conditioner, no matter its size, has a BTU rating. Just looking at a BTU rating will not help you select the appropriate air conditioner for your home. You must measure your home to determine the necessary BTU requirements, and then you can select a unit that will meet your room's needs.
Measure the length and width of the room with a tape measure. To obtain the square footage, multiply the length by the width.
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Multiply the square footage of the room by 25-BTU. According to the Aubuchon Hardware website, each square foot requires 25-BTU of cooling.
Add 1,000-BTU for each window in the room to account for cooling loss.
Add 400-BTU for each person who routinely uses the room. For example, if you're cooling a bedroom that two people use, add 800-BTU. If you are cooling a living room that four people use, add 1,600-BTU.
Add the BTUs obtained in Steps 2 through 4 to obtain the total BTU output needed.