A BTU is a unit of energy, and for homeowner's purposes it is used to measure the cooling potential of an air conditioner. To figure out the size of the air conditioner that you need to cool a room in your house, you need to know the area of the room. Once you have that, since the Department of Energy recommends 20 BTUs for each square foot of space, it is an elementary calculation.
Things You'll Need
 Tape measure
 Calculator

Measure your room's width. Make the measurement in inches for the most precision.

Measure your room's length, in inches.

Multiply the length in inches by the width in inches to get the total area. For example, if your room is 180 inches by 240 inches, the overall area square inches is 43,200.

Divide the area in square inches by 144 to get the area in square feet. In this example, 43,200 translates to 300 square feet.

Multiply the number of square feet by 20 to get the needed BTUs for the room. If your room is 300 square feet in area, you need an air conditioner rated at 6,000 BTUs.
Tips & Warnings
 If you are in an especially warm area, or if your house is exposed to a lot of direct sunlight, adjust upward the 20 BTU per square foot figure in your calculation. Conversely, if your house is shaded by trees or other objects that block direct light, lower the 20 BTU figure.
 Don't buy a bigger air conditioner than you need. You will pay more for the unit and for the power to run it, and it won't cool your house any better than a unit of the proper size.
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References
 Photo Credit air conditioners mounted on the walls image by Sergey Kolesnikov from Fotolia.com