To determine the size of air conditioner needed in a space, first measure the square footage of the area that needs to be cooled. Determine how many British thermal units the air conditioner has to produce per hour to cover the square footage. Make any necessary adjustments for special conditions, such as cathedral ceilings, heavy shaded rooms or rooms that receive direct sunlight.
To calculate the square footage of a rectangular room, multiply the room's length by its width. Multiply the area of a rectangular room by the width, and then divide by two. To find the needed Btu for a space, consult a chart, such as the one published by the U.S. Environmental Protection Agency. For example, a chart may recommend a unit with a 12,000 Btu capacity for a space with 450 to 559 square feet.
For a space that is shaded from direct sunlight, adjust the BTU capacity by 10 percent, and vice versa for a space that receives direct sunlight with no shading. Add 600 Btu for each person who resides or occupies the space. If the unit is for a kitchen, increase the Btu capacity by 4,000 to counter the heat generated by oven, stoves and other appliances. For particularly large spaces, consider using multiple units.