How to Properly Size a Window Air Conditioner

Don't buy a more powerful unit than you need

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illustration of living room with window ac, blue arrows show width and length of room Illustrations: T.M. Detwiler

Size matters when you’re buying a window air conditioner.

An air conditioner that’s too small will struggle to keep a room at a comfortable temperature. A model that’s too big will cool a room too quickly without removing enough humidity from the air, leaving you cold and clammy.

More on Cooling

Choose just right and you’ll feel just right—and save money, too. Consumer Reports tests air conditioners in rooms that are the same size as the ones they’re intended to cool. That makes it easier for you to select the best model for your needs.

After installing a window air conditioner in a double-hung window in our lab’s climate-controlled chamber, we crank the air temperature up to 90° F and measure how long it takes the AC to cool the room by 10° F.

“The best models in our tests can cool the room in less than 15 minutes,” says Chris Regan, the engineer who oversees CR’s air conditioner tests.

We also gauge how accurately the AC reaches the set temperature, whether each model can recover after a simulated brownout, how intuitive the controls are, and how loud each unit is while running on low and high.

What Size Air Conditioner Do You Need?

Bedroom or home

sq. ft. 100 to 250
btu 5,000-6,500

Primary bedroom or

sq. ft. 250 to 350
btu 7,000-8,500

Living room, family room,
or open plan

sq. ft. 350 to 550
btu 9,800-12,500

The Rules for Keeping Cool

Window air conditioners typically have a cooling capacity ranging from 5,000 to 12,500 British thermal units (Btu/hr.). As a rule of thumb, an air conditioner needs 20 Btu for each square foot of living space.

But other considerations, such as the ceiling height and the size of your windows and doorways, might call for more cooling power.

To measure your room, multiply the length by the width. Add together the size of rooms that aren’t separated by doors, because the air conditioner will need to cool both spaces. Energy Star recommends that you make adjustments for the following circumstances:

• If the room is heavily shaded, reduce capacity by 10 percent.
• If the room is very sunny, increase capacity by 10 percent.
• If more than two people regularly occupy the room, add 600 Btu for each additional person.
• If the unit is used in a kitchen, increase capacity by 4,000 Btu.

Best ACs for Small Rooms

For a small bedroom, home office, or guest room, ranging from 100 to 250 square feet, look for an air conditioner with a capacity of 5,000 to 6,500 Btu. You’ll want one with good scores for comfort and noise. Here are three top-rated models.

Best ACs for Midsized Rooms

To cool a bigger or busier room ranging from 250 to 350 square feet, you’ll want to step up to a midsized air conditioner rated at 7,000 to 8,500 Btu. If it’s for a bedroom, heed our noise scores. Here are three top picks.

Best ACs for Large Rooms

For a living room or family room of 350 to 550 square feet, you’ll want a large air conditioner with 9,800 to 12,500 Btu, especially if you have an open floor plan. Here are three solid choices.

Money-Saving AC Tips

Tougher federal energy standards for window air conditioners make newer models cheaper to run. Look for a unit with an Energy Star label and an energy-efficiency ratio (EER) of 10 or above. The higher the EER, the lower your operating costs. All the models in CR’s air conditioner ratings have an EER of 10 or higher.

Keeping your AC in peak condition can also help you save. Look for a model with a filter that’s easy to remove for regular cleaning. A dirty filter causes an air conditioner to work harder.

Many air conditioners have a timer that you can set to cool a room before you get home. And some have an app for your smartphone that you can use in case you forget to do it before you leave the house.

Maintaining Your Air Conditioner

We rely on our window air conditioners to keep us cool when temperatures climb. On the “Consumer 101” TV show, Consumer Reports expert John Galeotafiore explains to host Jack Rico how to maintain a window AC unit to ensure cool, clean air throughout summer.

Mary H.J. Farrell

Knowing that I wanted to be a journalist from a young age, I decided to spiff up my byline by adding the middle initials "H.J." A veteran of online and print journalism, I've worked at People, MSNBC, Ladies’ Home Journal, Good Housekeeping, and an online Consumer Reports wannabe. But the real thing is so much better. Follow me on Twitter.