How to Properly Size a Window Air Conditioner

Mary H.J. Farrell
·3 min read

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Size matters when you’re buying a window AC.

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.

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
office

sq. ft. 100-300
btu 5,000-6,500

Primary bedroom or
playroom

sq. ft. 250-400
btu 7,000-8,500

Living room, family room,
or open plan

sq. ft. 350-650
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.

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 above.

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 of the newer connected models have apps that enable you to adjust settings remotely from your smartphone.

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.