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Heating and Cooling How to determine what size window air conditioner you need

Published on June 6th, 2013 | by Terry Spencer


How to determine what size window air conditioner you need

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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About the Author

Terry Spencer is an in-house appliance and HVAC expert for He has 29 years of expertise in appliances and HVAC equipment repair. Prior to joining RepairClinic in 2011, Terry was a respected, well-known technician at one of metro Detroit’s top residential appliance and HVAC repair businesses. Enthusiastic about fixing things and helping people, Terry enjoys his position with RepairClinic. He has a leading role in the development of RepairClinic’s how-to repair videos and repair help content. He’s also frequently called on for technical advice and press information for journalists and bloggers. He regularly completes extensive manufacturer training for appliances, HVAC equipment and outdoor power equipment. Terry has a refrigeration license and holds multiple certifications for an array of products. As a child, Terry was known to take appliances apart simply to see how they worked. While growing up on a farm, he taught himself how to repair small engines. Terry enjoys creating content that reaches and helps so many people solve problems on their own. In collaboration with colleagues, he develops and leads frequent Customer Care staff training programs for appliances, HVAC equipment and outdoor power equipment. When he’s not at RepairClinic, Terry spends time with his wife and children. An avid outdoorsman, Terry also enjoys Michigan’s seasons. He and his family reside in the Ann Arbor, Michigan area.

4 Responses to How to determine what size window air conditioner you need

  1. Michelle L says:

    Thanks so much for the valuable info! Looks like temps are going to be scorching around here again this summer & we’re in need of a new window A/C.

  2. Dave says:

    Is there an easy way to prepare a vinyl replacement window to install a window air conditioner? Since the bottom of the window is no longer flush with the sill I usually end up clamping a 2X4 to the sill and filling in btw the inner & outer vinyl with wood also to support the unit plus more outside the window. It would be nice if some one would make some kind of bracket/support device to do this.

    • Hi Dave,

      Thanks for the comment. Yes, there is a great bracket to solve that common dilemma of properly supporting AC units in windows. It installs from the inside so it’s simple to set up. There are two sizes based on the AC unit size. On, you’ll find item #12915, which supports up to 85 pounds (usually an air conditioner of 5,000 to 10,000 btus).

      See it here:

      For larger units, up to 160 pounds (usually air conditioners from 12,000 to 24,000 btus), item #12916 comes with its own bubble level for more accurate installation. If you have any questions about these brackets, please contact us at 1-800-269-2609. Thank you! Have a nice summer.

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