Informational Guide

What Size Window AC Do I Need?

Getting the right sized AC unit will mean more efficient cooling. We show which window air conditioner you need with our size guide.

by Josh M

An air conditioner can be a real asset to your home, but only if you get the right one. There’s a lot to consider when you’re searching for the right AC, and you need to consider the type, model, and size of your unit.

Window air conditioners are popular with many homeowners, but they rely on the dimensions being correct so you can feel the benefits. You may well be asking, ‘what size window AC do I need?’

In this guide, we’ll help answer that question and explain how to choose the correct size of window air conditioner.

Your air conditioner is designed to regulate the humidity in your home. They do that by monitoring the moisture levels in the air and removing it if needed. This helps to provide a cooling effect and gives you a well-ventilated home.

The size of your AC should relate to the dimensions of your home. The output from an air conditioner is measured in British Thermal Units (BTUs), and this is an indicator of how much of an impact your AC will have. The higher the BTUs, the greater the cooling effect the AC will have in your home.

If your AC is too small for your home and doesn’t have enough BTUs, then you won’t get an even temperature or humidity. It can lead to cold spots in your home, and you won’t get the true benefit of the device. Your air conditioner will also run for longer and work harder to try to get the desired effect which can increase your energy bills and even cause damage to the unit.

If your AC is too large, it can lead to short cycling. This happens when your AC cools your home too quickly, and it continually turns on and off. This can damage the internal components and impact the longevity of the machine. Larger ACs are also more expensive to run, and you will end up paying higher energy bills.

It’s essential that you get the right size window AC to maximize the benefit and minimize the energy costs. You also need to consider the physical dimensions and get an air conditioner that will actually fit in the space. Obviously, if your window is only 36 inches wide, you can’t install a 48-inch AC, so getting the right size is vital. That’s why you can’t skip this step in the process, and you need to make sure you size it correctly.

Split Air Conditioning Units Across Fire Exit

How To Size a Window Air Conditioner?

It’s important to understand the key factors which will influence how big an AC you’ll need. There’s a lot to consider, but here’s a list of the main things to look out for:

Room Size

The size of your room impacts the amount of cooling you’ll need. A large room will need more BTUs, and a smaller room won’t need as many. A more powerful AC will cost more, and it can be tempting to go with a cheaper option, but don’t. The window AC you go for should be influenced first and foremost by the size of the room it’s operating in.

Number & Size of Windows

For a window AC, you will need at least one window, but it’s worth considering the shape and size. You can get modern window ACs that will fit in a range of shapes, but you’ll need to measure it carefully before buying one.

If you have a room with several windows, it might also mean that the room is poorly insulated with a lot of heat transfer. You’ll potentially need a more powerful AC to compensate.

Insulation

Many people think insulation is just to keep the heat in, but it’s actually used to stop heat transfer. This means it can keep a room cool or hot depending on what the conditions are. If you have insulation, it should make it easier to maintain a consistent temperature, and your window AC doesn’t need to be as powerful.

Depending on the quality of your insulation, you may be able to get a window AC with 10-15% fewer BTUs.

Shade or Sun

The amount of shade or sun hitting a room will influence the temperature and how long it takes to heat and cool. If the room with the window AC is shaded, you should reduce the number of BTUs by about 10%, and if it’s sunny, you should increase the BTUs by 10%.

Occupancy Level

People give off a lot of heat, and we’re basically walking hot water bottles. If you have more than 1 person in a room for prolonged periods of time, then your AC will need to work harder to cool the area. If you’re going to have 2 people in a room regularly, you should add about 600 BTUs. If there’s more, then you should add an additional 600 BTUs for each of them.

Ceiling Height

The size of AC you need is usually determined by square footage. The higher your ceilings, the more space there is to cool and the more BTUs you’ll need. Many people consider the length and width of the room, but you need to think about the ceilings too to get the best window AC. If your ceiling is over 8 feet, then you should add 10-15% extra BTUs.

Electrical Outlet

Almost all window air conditioners are electrical and need to be plugged in. This means you’ll need an electrical outlet nearby as it’s not recommended that you use an extension cord. You’ll typically need a 120V outlet, but it’s worth checking the individual electrical requirements as some larger window ACs will need more.

You can get your window AC hardwired directly into your electrics, but this is much more expensive and will need professional input.

What Size of Window AC Do I Need? (Guide Based On Room Square Feet)

Window ACs are typically not designed for huge spaces and will only cool one room or area. The size of the AC you need is measured in BTUs, reflecting how much power it has. The more BTUs a window AC has, the larger the area it can cool.

It’s important that you get the most appropriate AC for your space. We’ll explain how to accurately measure your room in a second, but here’s a rough breakdown of the size of AC you’ll need based on the area it will be operating in. Remember, you will need to consider the other factors we discussed above because this could dramatically change the window AC size you need.

Room Area To Be Cooled Window AC Size (BTU)
100 to 150 sq ft. 5000
150 to 250 sq ft. 6000
250 to 300 sq ft. 7000
300 to 350 sq. ft 8000
350 to 400 sq. ft. 9000
400 to 450 sq. ft. 10,000
450 to 550 sq. ft. 12,000
550 to 700 sq. ft. 14,000
700 to 1000 sq. ft. 18,000
1000 to 1500 sq. ft. 24,000

Calculating The Rooms Square Footage

As you can see, the dimensions of your room will hugely influence the size of window AC you buy. You need to calculate the room’s square footage accurately, or you could end up with a useless air conditioner. The good news is that it’s pretty straightforward, and you can do it yourself:

Square & Rectangular Rooms

Square and rectangular rooms are the most straightforward. You need to measure the length and the width of the room and multiply them together to get the square footage. Make sure you’re measuring in feet and not meters, and use a measuring tape (don’t just eyeball it). Here’s a quick example:

The room is 12 feet long and 10 feet wide.
12 x 10 = 120
Therefore, the room is 120 square feet.

Triangular Rooms

Triangular rooms are slightly more complicated, and the equation is slightly different. First, you’ll need to measure one side of your room, and then from the center of that side all the way to the far wall. This will give you the base length and the height length. You then need to do ½ x base x height to work out the area. Here’s an example:

One wall is 12 feet long and the distance from the middle of this wall to the other wall is 20 feet.
0.5 x 12 x 30 = 180
Therefore, the room is 180 square feet.

Size of Window AC For Different Room Types

To save you some time, here are the rough breakdowns for these different types of rooms and what size AC you might need. Remember, always measure accurately yourself because these are just a guide:

lg window ac
Bedroom

The average bedroom is about 150 square feet, so you’ll want around a 5000 BTU window AC. If you have several people in the same room, you might want a bit of extra power, so 6000 BTU could be better.

Living Room

The average living room in the US is between 300 and 500 square feet. For this kind of space, you’ll need an 8000-11,000 BTU window air conditioner, but you may need slightly more depending on the number of windows and other insulation in the room. If you have a large household with a lot of people using the living room, you’ll need a window AC of around 10,000-14,000 BTU.

Apartment

Your average apartment is around 1000 feet, though you can get some that are a lot larger. A window AC will need to be roughly 18,000-20,000 BTU to cool this space, but if your apartment is split into several different rooms, then you might need several air conditioners.

Garage

A one-car garage is roughly 100 square feet, but because they generally have poor insulation, they can get hotter than the rest of your house. You’ll typically need an AC of around 5000-6000 BTU to cool it. For a two-car garage, you’ll want an 8000-10,000 BTU window AC.

Kitchen

Your kitchen is unique in your house because there’s a lot going on. The cookers and other appliances all add a lot of heat to the room, and it’s recommended that you add about 4000 BTUs on top, so you have adequate cooling. The average kitchen is about 100-300 square feet, and you’ll want a 10,000-13,000 BTU window AC to cool it.

How To Measure A Window For An Air Conditioner

Alongside the BTUs of an air conditioner, you also need to consider the physical dimensions and whether it will fit in your window. It’s important to measure your window correctly so you can find an AC which can be installed properly. Here’s how to accurately measure different types of windows:

1. Casement Window

A casement window is attached to the frame by hinges at the side, top, or bottom. You’ll need to close the window and then measure the entire length of the base of the window, even if it’s separated into several smaller windows. Then you should measure the entire height, so you have both measurements. Double-check the measurements and make sure they’re in inches.

2. Sliding Window

These windows slide open, so start by making sure the window is completely closed. Then, measure along the bottom of the window and then the top to the windowsill. Make sure you go right to the edges and then double-check the measurements.

3. Single Hung Window

A single hung window opens by sliding half of it up vertically. Open up the window entirely and then measure the space. The window AC will likely sit in the half that opens, so knowing the height and width of this area will determine what size of AC you need. Make sure you go right to the edges and double-check your measurements.

4. Double Hung Window

A double hung window is split into two halves, both of which can slide up and down. The easiest way to measure this is to close it up and then measure the height and width of the whole thing. Then measure the height and width of each separate panel. You can then look for an AC that fits the entire space or just in one half of the double-hung window.

5. Louver Window

A louver window has an individual horizontal panel that opens up like a vent. Close up all the louvers and then measure the height and the width of the whole thing. Then double-check the measurements.

Jeacent AC Window Air Conditioner Support Bracket

People Also Ask (FAQ)

What size room will a 5000 BTU window AC cool?

This will effectively cool a 100-150 square foot space, depending on the insulation and other factors.

Will higher BTU cool faster?

Yes, the higher the BTU, the more powerful the AC is and the more quickly it will cool an area.

Can Window AC units cool the entire house?

Window ACs are only really designed to cool a single room, so they shouldn’t be used to cool your entire house. They can be effective in apartments, though.

Does higher BTU mean more electricity?

Generally, yes, the more BTUs the AC has, the more power it will use to run. It’s worth looking at the energy efficiency rating of your window AC, as an efficient unit can save you a lot in the long run.

Conclusion

Window air conditioners come in all different shapes and sizes, but it’s important you choose the right one for your home to maximize the benefit. Hopefully, this guide has helped explain how to size a window air conditioner and will allow you to find the best AC to cool the area.

Josh M

My name is Josh and I am obsessed with the HVAC industry. I created this website to help HVAC techs of all levels get the best out of their heating & cooling systems. I have spent thousands of hours studying air conditioners, heaters and home air products so you can learn & buy with confidence. Learn more about the team here.

Top