#### What You'll Learn

One Size Doesn't Fit All
Matching the air conditioner to the room you want to cool is your first consideration. If it's too small, it simply won't cool the space. If it's too large, it will cool the space so quickly that the thermostat will shut the unit down before it can lower the humidity level in the space. The result is a cool room that feels damp and clammy.

Room air conditioners are "sized" according to their cooling capacity, expressed in Btu per hour. You'll find the rating on packaging or in product literature; it typically ranges from 5,000 to 25,000 Btu. Units below 12,000 Btu start at \$250 to \$300. Those in the 12,000- to 15,000-Btu range cost from \$450 to \$600, while large units go up to \$1,200. Air conditioners rated at 15,000 Btu should be enough to cool most rooms up to about 875 sq. ft. How do you get more precise than that? There are several different formulas. Most of the ones you'll see in stores simply compare the square footage of the room to Btu capacity. The Association of Home Appliance Manufacturers (AHAM), a Chicago-based industry trade group, offers a more complete formula that includes window area, sun exposure, insulation levels, how the room is used and other variables.

How Much Do You Need?
For a calculation of the cooling capacity your space requires, use our Air Conditioning Calculator.

You can get a free copy of the formula from the AHAM fax-on-demand service or through the organization's site on the Web. Although the form looks daunting, it takes only a few minutes with a calculator and tape measure to figure out the square footage of the room and measure a few other variables. Whichever way you determine the size you need, remember that for the 115V circuits typical of bedrooms and living rooms, you'll have to stay below 15,000 Btu; units above 15,000 Btu usually require a 230V circuit. And if you want to cool a large, open L- or T-shaped space, you're better off with two smaller units rather than one large one. "The goal is to keep the cold air from pooling on the floor," explains Hank Rutkowski, director of technical affairs for the Washington, D.C.-based Air Conditioning Contractors of America. "Two units prevent cold spots in front of the air conditioner and warm spots around the corner."