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If you’re looking to buy a window air conditioner, the most likely question you have is, “What size window AC unit do I need?”

And that’s a good question to ask.

The last thing you want is to buy an underpowered air conditioner that can’t get the job done or spend a larger sum of money on an AC unit that’s too powerful for the room.

That’s why we wrote this post.

To answer the question, “What size window air conditioner do I need?”

By the end, you’ll have everything you need to figure out how to get the right size window unit for any size room.

The Elements Of A Good Window Air Conditioner

Before we jump into the process of sizing a window air conditioner, we first need to go over the importance of selecting a good window AC unit.

The fact is that many of us tend to focus too much on nailing the right measurements for a window unit and tend to ignore other important factors that go into a high-quality air conditioner.

To ensure you don’t make the same mistake that countless others have made, we suggest that you take a look at our guide on the top rated window air conditioners.

It goes over all of the key features you need to look for in a device that you’ll be happy with.

For the purpose of this post, we’ll pull out three things that apply:

Air Filter Type

By design, window air conditioners suck hot air in from a room, cool it down, and then pump cold air back into the space.

Unfortunately, no matter how clean your home may be, the air around you is full of microscopic particles that can damage the internal components of the window AC unit (e.g. dust, hair, pollen).

Therefore, all window units come with an air filter to trap those particles and prevent them from entering into the machine.

Additionally, indoor air can also be full of pollutants like mold spores, bacteria, and allergens.

An air filter on a window AC unit can also help remove those contaminants from the room.

When shopping for a window air conditioner, you’ll find that there are two types of air filters:

  • Standard filter
  • Antimicrobial filter

Both filter types consist of a mesh screen to trap airborne particles; however, the antimicrobial version has an additional layer of protection to prevent the growth and spread of bacteria and other harmful contaminants.

When buying a window AC unit, it’s always best to get one that includes an antimicrobial filter.

It not only protects the internal workings of your window unit but also creates cleaner air for you to breathe.

Energy Efficiency

Another good tip is to always select a window air conditioner that has the Energy Star label.

This certification indicates that the unit uses a low amount of energy to operate.

Therefore, you won’t have to worry about opening the electricity bill at the end of each month.

Thermostat Type

Depending on your budget and the type of window air conditioner you’re looking to buy, you’ll find that there are two types of thermostats:

  • Manual thermostat
  • Digital thermostat

A manual thermostat looks like a dial and gives you relative control of the cooling temperature.

The dial will have numbers that pertain to the coldness of the air, like 1-7, with 7 being the coldest temperature.

A digital thermostat has a digital readout of the temperature on an LCD screen.

It also includes buttons for fine-tuned control. You can be precise in the temperature setting down to a specific degree.

Clearly, a digital thermostat is the better choice, but you do pay more for this luxury.

See all of Amazon's Best Selling Window AC Units

What Size Window AC Unit Do I Need?

Now that you have a few buying tips under your belt, let’s get into the real meat of this post by answering the question “What size window air conditioner do I need?”

The Importance Of Measuring The Room

When it comes to determining the proper size of a window unit, you’ll first need to know whether it’s the right for the space.

If your window air conditioner is too big for the room, it’s going to cycle irregularly, something which will bring you unnecessary expense. 

Conversely, if the appliance is too small, it won’t cool the room sufficiently.

For those reasons, you need to figure out the square footage (sq. ft.) for the room you’re going to use the window air conditioner in.

How to Get the Right Size Window Air Conditioner

Calculating a Room’s Square Footage

In this guide, we’re going to cover the two most common types of rooms that you’ll come across in your home: square/rectangular and triangular.

  • Square & Rectangular Rooms — Measure the width and length of the room in feet. Then, multiply those two numbers together.
    • Example: 20 feet x 22 feet = 440 sq. ft.
  • Triangular Rooms — Measure the width and length of the room in feet. Then, multiply those two numbers together. Finally, divide in half.
    • Example: 20 feet x 22 feet = 440, and 440 ÷ 2 = 220 sq. ft.

For oddly shaped rooms, just do the best that you can by breaking the room down into smaller squares or triangles. Get the square footage of each of these smaller sections and then add them all together.

BTU is a Crucial Component for Size

This is the step most people skip and what leads to buying an underpowered or overpowered AC unit.

Once you have the square footage number in hand, you need to pair it up with the correct BTU measurement on a window air conditioner.

BTU stands for British Thermal Unit and it’s the international standard for measuring thermal (heat) energy.

A window air conditioner’s BTU quantifies the size room it can handle. The smaller the number, the smaller the room it can cool.

Conversely, the higher the number, the larger the room it can cool.

Here’s an easy-to-read chart to help you pair up your square footage to the proper size BTU number.

Once you find the match, you can go shopping for a window air conditioner that meets or exceeds this BTU measurement.

Square Footage to BTU Chart

Room Area To Be Cooled (square feet)Capacity Needed (BTUs per hour)
100 up to 150 sq. ft.5,000 BTU
150 up to 250 sq. ft.6,000 BTU
250 up to 300 sq. ft.7,000 BTU
300 up to 350 sq. ft.8,000 BTU
350 up to 400 sq. ft.9,000 BTU
400 up to 450 sq. ft.10,000 BTU
450 up to 550 sq. ft.12,000 BTU
550 up to 700 sq. ft.14,000 BTU

Measure The Size Of the Window

A final thing you’ll want to consider when asking yourself the question, “What size window AC do I need?” is the size of the window you’re going to place the unit in.

Obviously, not all window air conditioners can fit in every size window; bigger window AC units required wider windows.

Also, window air conditioners don’t necessarily span the entire width of your window.

That’s why these units come with accordion side panels to cover up the side gaps. 

However, side panels do have a maximum length for which they can stretch (mentioned in the product specifications).

Therefore, you want to measure the inside of your window casing so you can ensure that the air conditioner you buy can actually fit inside, and the side panels can stretch enough to cover the gaps.

Next Steps

Now that you know the answer to the question, “What size window AC unit do I need?”, you’re likely wondering what the top products are you can get for your house.

To help aid you in your purchase, we’ve put together a detailed guide on the top rated window air conditioners.

It explains everything you need to look for in a high-quality appliance as well as offers a few recommendations for each size room. So, take a look.

If you’re getting a window air conditioner for a small room, you may be interested in knowing what your other options are besides window units.

If that’s the case, you’ll find our options for the best AC for small rooms to be a good read.

We hope this guide on what size window air conditioner do I need was informative and helped you feel more confident in choosing the right AC unit for your needs.

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About David Morrison

David is an Air Quality & Comfort Technician. He has expert knowledge on the technology and design of air purification, air conditioning, and heating systems. His main role is to write content that helps people get the most value out of their air purifiers, air conditioners, and heating units. (See Full Bio)