When you start shopping for an air conditioner for your home, you quickly discover you have to make some decisions. Do you want a split-system unit or a packaged air conditioner? Or are you aiming for something to cool a targeted room, like a portable air conditioner or a ductless air conditioner? Answering all of these questions comes down to one thing: the right air conditioner size for your home.
By size, we’re not talking about the area the air conditioner itself will take up. Instead, we’re talking about its capacity. Sizing your AC unit correctly ensures your home can stay adequately cool without working your system overtime or paying for more air conditioner than you need.
Generally, sizing your AC correctly hinges on making sure it has a great enough cooling capacity to cool every square foot of your home (for central air conditioners) or specific room (for mini-split ductless air conditioners). But there are special considerations, too. Obviously, a room that gets a lot of sun is going to need an AC unit with more cooling capacity than one that doesn’t take on much heat throughout the day.
Good news: You can have an HVAC professional come out and precisely size an AC unit for you. But doing some basic calculations on your own before you get a pro involved positions you as an educated shopper. By having a basic idea of what you need before you start talking to HVAC contractors and dealers, you can rest easy knowing your home’s actual needs — not a specific preference of the vendor — dictated the size of your AC unit.
So let’s get started.
In this article:
- Factors that Influence Size
- How to Determine Central Air Conditioner Size
- How to Determine Ductless Mini Split Air Conditioner Size
- Why Does the Central Air Conditioner Size Matter?
- Tips for Sizing a Central Air Conditioner
Factors that Influence Size
Here are the things you need to think about when you’re sizing your air conditioner.
First things first, how much of your house do you want to cool? If you’re hoping to bring down the temperature throughout your entire house, you’ll want a central air conditioner, which connects to your ductwork and recirculates cooled air through every room.
But if you only have a single room you want to chill, you may want a mini-split ductless system. This air conditioner gets mounted high on the wall in the room and delivers cooling for that specific room. You can also get multiple ductless air conditioners for different rooms.
Cooling capacity (in BTUs or tons)
When an air conditioner’s cooling capacity is measured, the metric used is British thermal units (BTUs). Your AC unit might not give its cooling capacity in BTUs, though. Often, capacity is described in terms of tonnage. Here, one ton is 12,000 BTUs of cooling per hour.
The U.S. Department of Energy gives a couple of guidelines for sizing your air conditioner. If your home is older or you live in a hot climate, you should generally allow for one ton of cooling for every 400-500 square feet you want to cool. But if you have a well-sealed, energy efficiency home, you may only need one ton of cooling per 800-1,000 square feet.
You can also use this ENERGY STAR AC sizing table to help you determine how many BTUs of cooling capacity you need.
Your air conditioner doesn’t just pull warm air from the room, remove heat, and then recirculate the now-cooled air back through your space. As it works, EnergyStar.gov says air conditioners also remove moisture from the air. That means you get less of that sticky humidity in your home.
Here’s where sizing comes into play on the top end of the spectrum. If you get an AC unit that has a much greater cooling capacity than you need, it only has to run for a short time to cool your space. While that delivers quick cooling, it means less air is getting circulated through the system. And that means more moisture can hang around in your house. Not ideal if you’re trying to avoid humidity.
Sun and heat exposure
Obviously, every room in your house doesn’t stay at the same temperature. Sun-facing rooms warm quickly and the kitchen can heat up fast when you’re cooking. If you’re going to get a ductless mini-split air conditioner — which only cools the room in which it’s located — it’s important to factor these things in. We’ll get into more details in the “How to Determine Ductless Mini Split Air Conditioner Size” section below.
Bonus: energy efficiency
While this technically doesn’t affect the size of air conditioner you need, we recommend also checking the energy efficiency of your potential future air conditioner. For AC units, energy efficiency is marked as a seasonal energy efficiency rating (SEER). The higher the SEER, the more efficient your unit will be. And that means lower utility bills whenever you run your air conditioner.
How to Determine Central Air Conditioner Size
If you’re choosing a central air conditioner — an air conditioner that connects to your home’s ductwork and circulates cool air throughout your house — calculating a ballpark AC size will require you to get out your tape measure. Here’s a step-by-step guide.
1: Measure the area of every room you want to cool
There may be times where you plan to close the ducts to all rooms but one or two. But when it’s hot out, you most likely want to have the option to cool every room in your house. Measure the area (width times length) of every room you’d want to cool at the same time. That might mean skipping your attic or craft room, but make sure you measure the area of all bedrooms, bathrooms, living spaces, and the kitchen. Total up the area.
2: Know your home — and calculate accordingly
Is your home drafty with single-pane windows? Or have you sealed it and fitted it with energy-saving features? If it’s the former, the U.S. Department of Energy suggests you divide the total area you want to cool (your calculation from step #1) by 400. The resulting number is the number of tons of cooling capacity you likely need.
But if your home is energy efficient (i.e., well-sealed and well-insulated), you can divide your total area by 1,000 to find the number of tons of cooling capacity you’ll need.
Just remember, air conditioners might report their cooling capacity in terms of BTUs rather than tons. To find the cooling capacity in BTUs, just multiply the tonnage by 12,000.
3: Confirm your calculations with a pro
A central air conditioner is a sizeable investment. It’s important to have a ballpark size in mind before you start talking with a pro so you can be an informed shopper, but you shouldn’t necessarily purchase your unit without having a seasoned professional confirm that you’re looking at the right AC size for your home.
Ask local HVAC contractors and dealers if they offer a Manual J calculation. You’ll probably have to pay for this evaluation, but it ensures your AC unit is sized perfectly. Or, if you’re lucky, your local utility company might include a Manual J calculation as part of a free energy audit. Call and ask.
During a Manual J calculation, the HVAC pro or utility provider looks at not just your home’s square footage, but also your climate zone, how many windows you have, how efficient those windows are, how much sun/shade specific rooms get, how many heat-generating appliances are located throughout your house, how many people regularly use each room, and your ductwork. Analyzing all of these factors allows them to arrive at an extremely accurate calculation of how much cooling capacity your specific home needs.
How to Determine Ductless/Mini-Split Air Conditioner Size
If you’re choosing a mini-split or ductless air conditioner — an air conditioner that’s wall-mounted and only cools the room in which it’s located — calculating size is a lot simpler. You can use these three steps as a guide or reach out to a pro for help.
1: Measure the room’s area
Get out your tape measure and measure the length and width of the room where you’re planning to place your ductless air conditioner. Multiply them together to get the room’s area.
If the room is connected to adjoining rooms by a wide opening that doesn’t have a door, calculate the area of those connected rooms and add it to your total area to be cooled.
2: Use this chart
Find your area below and you’ll see the cooling capacity you need. We pulled this chart directly from ENERGY STAR.
|Area To Be Cooled (square feet)||Capacity Needed (BTUs per hour)|
|100 up to 150||5,000|
|150 up to 250||6,000|
|250 up to 300||7,000|
|300 up to 350||8,000|
|350 up to 400||9,000|
|400 up to 450||10,000|
|450 up to 550||12,000|
|550 up to 700||14,000|
|700 up to 1,000||18,000|
|1,000 up to 1,200||21,000|
|1,200 up to 1,400||23,000|
|1,400 up to 1,500||24,000|
|1,500 up to 2,000||30,000|
|2,000 up to 2,500||34,000|
3: Adjust as needed
If the room you want to cool is shaded by a big tree or has very few windows, you can reduce the cooling capacity by 10%. If it gets a lot of sun, EnergyStar.gov suggests you could bump up your BTU calculation by 10%. If it’s a kitchen, add 4,000 BTUs to your total to accommodate for the heat-generating appliances. And finally, if more than two people use the room on a regular basis, add 600 BTUs for each additional heat-generating body.
Why Does the Central or Ductless Air Conditioner Size Matter?
If you choose an air conditioner that’s too small, it will have to run constantly to keep your space cool — and even then it may not keep temperatures comfortable. Running your AC unit 24/7 is a surefire way to drive up your utility bills or reduce the life of your unit.
On the other hand, if you choose an air conditioner that’s too large, you may pay more for a cooling capacity that you don’t need. But even if money is no object, an air conditioner that’s too large barely has to run to keep your home cool. That minimizes its ability to pull humidity from your air, which can leave your house feeling sticky.
When you’re choosing your air conditioner, taking the time to calculate the right size can keep more money in your pocket all while making your home more comfortable.
Tips for Sizing a Central Air Conditioner
Now it’s time to get sizing — and shopping. Here are our top tips:
- Get multiple quotes. Once you have a ballpark idea of the AC unit size and type you need, you’ll want to reach out to HVAC contractors and air conditioner dealers to start getting pricing on specific air conditioners. This isn’t the most fun to-do — but don’t skimp. Take the time to get quotes from multiple vendors to ensure you get the best price on your air conditioner. Also, as you collect quotes, take note of the service you get. Since your air conditioner vendor will likely end up being the one who services your unit for maintenance and repairs, finding a company you like can make life easier and more pleasant down the road.
- Get info on energy efficiency. Don’t just shop in terms of BTUs or tons. As you look at your options, check the SEER, too. A higher SEER keeps your energy bill as low as possible.
- Get a pro involved. When in doubt, get a Manual J calculation. At most, it’s a small upfront expense. It’s well worth it to ensure you’re choosing the right AC size for your needs.
Recap and What’s Next?
With these step-by-step guides and tips, you should have no problem sizing an air conditioner for your home. Now, it’s time to get it installed. But don’t just set it and forget it. Regular maintenance keeps your AC unit running as efficiently as possible, keeping your utility bills down. More money in your pocket and lower temps for your home? That’s cool.