Size matters when you’re buying a window air conditioner. Buy too small and it will struggle to keep the room at a comfortable temperature; buy too big and and it will cool down the room too quickly without removing enough humidity from the air. Buy just right and you’ll feel just right—and save money too. At Consumer Reports we test air conditioners in the size rooms that they're intended to cool to make it easier for you to choose.

Window air conditioners typically have cooling capacities ranging from 5,000 to 12,500 British Thermal Units (Btu/hr). But don’t buy by Btu alone. As a rule of thumb, an air conditioner needs 20 Btu for each square foot of living space, but there are other considerations, such as the height of your ceiling and the size of your windows and doorways, that may create a need 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; the unit will need to cool both. 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.

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/hr. You’ll want one with good scores for comfort and quiet. Here are three to consider:

Master Bedroom or Playroom (250 to 400 square feet)
To cool a bigger or busier room, you’ll want to step up to a mid-sized air conditioner rated at 7,000 to 8,500 Btu/hr. If it’s for a bedroom, heed our noise scores. Three top picks:

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/hr., especially if you have an open floor plan. Three good choices:

The Best Fit for Your Budget

Tougher energy standards for window air conditioners that went into effect two years ago 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 of the models in our air conditioner tests have an EER of 10 or above.

Keeping your AC in peak condition also saves. Look for models with filters that are easy to remove for regular cleaning—a dirty filter causes an air conditioner to work harder. Newer air conditioners have timers that you can set to cool down the room before you get home rather than run your AC the whole time that you’re away. And some of the newest connected models have apps that enable you to adjust settings remotely from your smartphone.