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What Size Window Air Conditioner Do I Need? Here’s How to Find Out


When the sun is out and it’s blistering outdoors, the temperature inside your house increases as well. The unbearable heat might make it difficult to relax or focus on your tasks, so installing a window unit air conditioner is a sure-fire way to cool off hot, humid spaces if central air isn’t viable. Window unit AC units are available in local malls and online, and most have reasonable price tags that won’t burn a hole in your pocket. You might even find yourself asking, what size window air conditioner do I need?

You must purchase an adequately-sized air conditioner for the room you’ll be placing it in. Smaller units have less cooling capability, and if you use one in a small room, no matter how hard the unit works, the area will never be cool enough.

On the other hand, if you choose a new window AC unit that is too large for the area, you will wind up paying for the cooling capacity that you don’t need and an AC unit that will cycle on and off too frequently.

How To Know What Window Air Conditioner Unit You Need

On a hot day, AC units can make the temperatures pleasant within your home, but only if the air conditioner is large enough to cool the area you want it to cool. Purchase the appropriate size air conditioner for your needs and ensure that it has installation kits to properly secure it to your window.

Measure Window Dimensions

Before purchasing a window AC unit, measure first the window AC dimensions. You can’t simply purchase a window air conditioner and hope it fits on your window sill when you install your AC. Estimating window AC width and height can save you a lot of trouble and expense in the long run.

Take measurements of the length and height of your window and jot them down. You may start searching at window unit sizes in inches you know roughly what size your window has.

Determine BTU

What Size Window Air Conditioner Do I Need? Here's How to Find Out

Window air conditioners have cooling capacities ranging from 5,000 to 12,500 British thermal units or Btu/hr. According to the U.S. Environmental Protection Agency, an air conditioner requires 20 BTU for every square footage of the living area. Listed below are the estimated window length for the estimated window width needed for every window AC based on their BTUs. 

It also includes the area per square foot these window AC can cool.

Room Sq FtBTU RequirementsEstimated Window Width
100 to 1505,000 to 6,000 BTU21-35 inches
150 to 2506,000 to 7,000 BTU21-35 inches
250 to 3007,000 to 8,000 BTU22-36 inches
300 to 3508,000 to 9,000 BTU22-36 inches
350 to 4009,000 to 10,000 BTU22-36 inches
400 to 45010,000 to 12,000 BTU23-36 inches
450 to 55013,000 BTU25-38 inches
550 to 70014,000 to 18,000 BTU27-42 inches

Other Factors That Affect Cooling of Window Air Conditioners

Aside from the cooling capacity of the window AC, other circumstances that can affect the room you wish to chill, as well as its surroundings, should be taken into account.

Window Count and Size

Windows account for between 25% to 30% of a home’s thermal transmission, so the amount of windows you have in your house is a critical factor in choosing a window AC unit. If the room you want to cool has many windows, or if the window size is huge, you’ll want a large capacity window unit to install.

Sun or Shade

The lack of sun exposure means there’s less heat for the AC unit to cool. Reduce the BTU requirements by 10% if the room is heavily shaded or if there are little to no windows. On the other hand, add 10% to the BTU requirements for rooms that receive full sun and no shade.

Height of the Ceiling

Ceiling height also affects your AC unit’s cooling capacity. A 10-foot-high ceiling equals 25% of the room volume, so add an additional 25% to your BTU requirements for every 10 feet.


Cooling a well-insulated area is quicker and less expensive to chill since the cold air is retained longer. Smaller A/C units will have to cycle on more frequently or may not be able to keep up with the cooling demand of uninsulated rooms.

Level of Occupancy

The number of people that occupies the space you’re planning on chilling is also an important factor to consider. If the room is shared by two or more people, you’ll need to add 600 BTUs per person.

Volts and Amps

The amperage rating of your outlet considerably impacts your unit’s ability to chill an area. Some lesser BTU units may use regular 120V, 15-amp outlets while units that need 230V/208V 60Hz or 110V/120V 60Hz with 20-amp outlets can’t be operated with it.

If your circuit uses a 14 gauge wire, you cannot simply replace your 15-amp circuit breaker with a 20-amp breaker. You’d have to build an entirely new circuit out of 12 gauge wire.

EER of the Unit

The energy efficiency rating, or EER, of air conditioners, can also be used to assess the cooling efficiency of a window air conditioner unit. Divide the BTU rating by the power output of the device to get the EE rating. For example, a 12,000 BTU AC with a power output of more than 1,200 watts has an EER of ten.

Final Note

When the weather begins to warm, we rely on our window air conditioners to keep us comfortable. When purchasing a new window air conditioner, size is important as it determines if your unit can easily fit in your window with its installation kit and if it can effectively chill your home. 

For other posts regarding aircon maintenance, use, and reviews, check out our site.

Frequently Asked Questions

How many BTU do I need for a 12×12 room?

A room is 12 feet wide by 12 feet long, or 12 x 12 equals 144 square feet. You’ll need at least an AC window unit with 5,000 BTU.

What size window AC do I need for a 12×12 room?

As stated above a 12×12 room will need a window AC unit with at least 5,000 BTU cooling power, and this type of AC usually needs an estimated 21-35 inches of window width.

What size room will a 12000 BTU air conditioner cool?

Using the 20 BTU per square feet rule, 12,000 BTU AC window units can effectively chill a 600 square feet space.

What to consider before buying an AC?

  • The number of rooms to chill – If you need to chill a large number of rooms, you should consider installing central conditioning.
  • The right unit – Choosing the most expensive or high-tech one can be tempting, but it’s important to stick to what your household needs.

JP Reyes

JP has been in the aircon industry for almost as long as he has been alive. As a child JP would help his tatay fix aircon units at their junk shop in Cavite. After graduating UP in the early 2000's, JP then started his own Aircon servicing business and within 5 years had 10 shops in 8 different cities. Fast forward to today and JP brings all his experience and expertise online to give readers trustworthy advice and reviews about Air-conditioning buying, servicing, cleaning and repair in the Philippines.

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