Size matters when you’re buying a window air conditioner. An AC that’s too small will struggle to keep the room at a comfortable temperature; a model that’s too big will cool the room too quickly without removing enough humidity from the air.

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.

More on Air Conditioners

How we test window ACs. After installing a window air conditioner in a double-hung window in our lab, we crank the heat up to 90° F in the surrounding area 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 brownout, how intuitive the controls are, and how loud each unit is on its lowest and highest setting.

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 don’t buy by Btu alone. Other considerations, such as the ceiling height and the size of your windows and doorways, might call for for more cooling power.

To measure your room, multiply the length of the room 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.

What Size Air Conditioner Do You Need?

Bedroom or home
office

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

Master 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

Match an AC to Your Room Size

Bedroom or Office (100 to 300 Square Feet)
For a small bedroom, home office, or guest room, 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 quiet. Here are three to consider:

• GE AHM05LW, $190
Amana AMAP061BW, $200
• SPT WA-6022S, $200

Master Bedroom or Playroom (250 to 400 Square Feet)
To cool a bigger or busier room, 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. Three top picks:

• Friedrich Kuhl SQO8N10D, $710
GE AHS08LX (Lowe’s), $250
• Kenmore Elite 77087, $390

Living Room or Family Room (350 to 650 Square Feet)
For a living room or family room, you’ll want a large air conditioner, 9,800 to 12,500 Btu, especially if you have an open floor plan. Three good choices:

• LG LW1216ER, $350
• SPT WA-12FMS1, $365
• Frigidaire Gallery FGRC1244T1, $470

The Best Fit for Your Budget

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 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 models 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 the room before you get home rather than running it while you’re away.

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