The BTU of Room Air Conditioners for the Size of the Room

Sizing Charts

• There are a variety of BTU sizing charts available at appliance stores and on the Internet. Alternative-Heating-Info, for example, offers a chart recommending 5,000 BTUs for room sizes between 100 and 150 square feet; and an additional 1,000 BTUs for each 50-square-foot increase above this range. At 450 square feet, the recommended BTUs jump to 12,000; between 500 and 700 square feet, 14,000 BTUs; and between 700 and 1,000 square feet, 18,000 BTUs.

Energy Star Ratings

• These ratings match those provided by the federal government's Energy Star website, which also offers advice on how to measure different room shapes. Energy Star recommends accounting for certain variables affecting the cooling demands of your room. If the room is shaded, for example, reduce the BTU rating by 10 percent. If it gets full sun, increase BTUs by 10 percent; add 600 BTUs if more than two people occupy the room on a regular basis; increase by 4,000 BTUs if you're installing the unit in a kitchen.

Online Calculators

• CSG Network offers a handy online calculator allowing you to input room dimensions and several variables, after which the site returns a recommended BTU range. The first variable is insulation -- the better the insulation, the lower the BTU rating (well-insulated rooms tend to maintain warmth as well as coolness). You can also tell the site whether the room has direct exposure to the sun, and whether or not it is a kitchen.

Manufacturer Recommendations

• If you want a more precise estimate, "Consumer Reports" offers a detailed online calculator, adapted from a worksheet published by the Association of Home Appliance Manufacturers. The online worksheet has you input room dimensions, geographical location, wall and door setup, window exposure, number of people who customarily use the room, ceiling height, hours of operation and other variables. Operating a window unit chosen with this rating system should adequately cool a room, but always keep in mind that humidity, the number of people present, open doors and windows, cooling settings and other factors affect the temperature the unit can actually maintain.