How to determine what size window air conditioner you need

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A properly-sized air conditioner unit will maximize energy efficiency, remove humidity in the air and ensure your space is cool and comfortable throughout the season. Tweet this tip.

An undersized air conditioner will have to work harder, waste energy and cost you more money. Tweet this tip.

Conversely, an oversized unit will cycle too quickly and may not run long enough to remove humidity in the air. The result? Sticky or clammy-feeling air. Tweet this tip.

A little homework will go a long way in ensuring you find the right size air conditioner to maximum efficiency in your space and save money.

How to determine what size window air conditioner you need

Here’s the easy way to determine what size air conditioner is best for your space and needs:

  1. Measure your window size.
  2. Check the power available near the window. Some larger air conditioners require a dedicated 220 volt circuit.
  3. Cooling capacity is measured in British thermal units (Btus) per hour. You’ll need to calculate the number of Btus required for your space. Use the handy chart below and consider the following factors:
  • If you have high ceilings or multiple windows in your space, you’ll need to increase required Btus slightly.
  • If your space is highly shaded, you can reduce the required Btus by 10 percent.
  • If there is abundant sunshine in your space, increase the required Btus by 10 percent.
  • If the space includes a kitchen, add 4,000 Btus. Due to cooking, kitchens generate more heat than other rooms.
  • Add 600 Btus per person who regularly spends time in the room or space.

Here are the recommend sizes listed by square footage of the area to be cooled. All figures are based on a room with its ceiling at eight feet. Tweet this.

Square footage of area to be cooled/capacity needed (Btus /HR)

100 to 150 = 5,000

150 to 250 = 6,000

250 to 300 = 7,000

300 to 350 = 8,000

350 to 400 = 9,000

400 to 450 = 10,000

450 to 550 = 12,000

550 to 700 = 14,000

700 to 1,000 = 18,000

1,200 to 1,400 = 23,000

1,400 to 1,500 = 24,000


After you have your air conditioner installed, check out these tips about getting your unit ready for the cooling season.

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Something not working right? has replacement parts and accessories for air conditioners including air filters, covers, remote controls, knobs, touch-up paint, side curtains with frames and many other products. Tweet this tip.

With your model number handy, head to to find parts recommended for your model.

Related info

Six simple ways to get your window or through-the-wall air conditioner ready for summer

AC maintenance tips

Five ways to save money and energy during heat waves

How an air conditioner works

How to find your air conditioner’s model number

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