What Size AC Unit Do I Need for My Home?

While buying a new AC unit for your home may seem pretty straightforward, there are many factors to consider. During your search, you may wonder, “What size AC unit do I need?” Unfortunately, the answer isn’t so cut and dry. The most basic details you’ll need for an exact calculation include the size of your home, what shape your insulation is in, and the climate where you live. With our tips and the advice of an air conditioner installer, you can feel confident when choosing the size of your next AC unit. 

What Is the Standard AC Unit Size?

When purchasing a new AC unit for your home, consult a local air conditioning installer to ensure your calculations fit your home cooling needs. Here are some of the most common AC unit sizes (in both BTUs and tonnage) based on the square footage of homes:

AC Unit Size by Type

Be sure to consider how its size varies by type of air conditioner.

Whole Home AC Unit: Indoors

Whole-home air conditioners are typically divided into indoor and outdoor units. The indoor units, also called air handlers, connect to a blower and must be positioned at a central point for ductwork. They tend to be the size of a traditional HVAC furnace, but dimensions vary based on their size and capacity. The more tons/BTUs the system is rated for, the larger the dimensions will be. Average AC sizes for brands like Trane, Carrier, American Standard, and Goodman are:

Indoor units are not entirely column-shaped, but these dimensions should help you plan for a new model or upgrade. Note that some air handlers are designed for utility closets and tend to be larger, particularly wider and deeper than other types of indoor units.

Whole Home AC Unit: Outdoors

Outdoor AC units contain condenser coils and release heat into the outdoor air. The size of outdoor units have relatively uniform dimensions based on their capacity or how much heat they can move.

Window AC

Window AC units must fit the size of the window. Smaller window units may be able to be supported or framed in an open window. Again, dimensions vary based on the capacity of the unit—and can vary significantly for window units—so measure and plan carefully. Since window AC units cool smaller spaces, they tend to be on the smaller side of the dimensions range, with large units reserved for special cases.

Ductless Mini Split

Ductless mini split systems are unique because they don’t connect to your home’s ductwork system. They also contain two essential components: the air handler and the heat pump. Check out the average dimensions for these units:

Air Handler

Heat Pump

How Do I Calculate the Size of My Ideal AC Unit?

The U.S. Department of Energy recommends that everyone looking to install a new AC unit insist that their professional conduct a Manual J calculation to determine the size of the unit necessary to cool their home. While it’s a complicated calculation, Manual J takes a few different factors into account. Square footage is one of the most important.

Your home’s square footage is one of the primary factors you should consider when thinking about the size of your AC unit. If your home has higher square footage, it will need a higher-tonnage AC unit to remove more BTUs. The Manual J calculation also requires you to provide the number of heat-generating appliances in the home, which tends to increase with square footage.

Factors That Influence Air Conditioner Size 

While square footage is a great place to start, there are other factors that come into play. For example, if you have a large home with a lot of windows, you may need a larger air conditioning unit to make up for the heat generated by the sun. Here are a few things to consider.

Climate Zone

According to the Office of Energy Efficiency & Renewable Energy, the United States has five climate zones defined by the temperature and humidity of each climate. The hotter and more humid your location’s climate, the higher the tonnage needed to power an air conditioner and cool your home.

Ductwork

The Manual J calculation takes the age and condition of your home’s ductwork into account. If your ducts are older or are in poor condition, they won’t work as efficiently as newer ones in better condition. 

Windows

The size and efficiency of your windows matter when figuring out what size AC unit you’ll need. If your windows don’t do a good job of preventing air leaks, that will put a higher strain on your HVAC unit. You can expect HVAC professionals to ask you to provide the number of windows in your home and whether they’re newer double-paned or older single-paned windows. 

Sun Exposure 

Where your home sits in relation to the sun affects how hard your AC unit has to work to cool the house. The more shade you have over your home, the easier it is for your HVAC system to keep your house from heating to uncomfortable temperatures.

Insulation

Insulation is a big factor in determining how efficiently your AC unit runs and the cost of your utility bills. If you run your air conditioner but your home isn’t well-insulated, the cold air will escape much faster, causing your system to work harder and driving up your energy bills.

Number of People in the Home

Yes, body heat plays a role in the Manual J calculation. Humans produce a lot of heat, and the more people you have moving around your home, the harder your system has to work to cool it down. 

Ceiling Height

Homes with higher ceilings are more difficult to cool because there’s more area for the air to circulate through. The higher your ceilings are, the more your AC has to work to cool all that extra space. 

Home Exterior

Believe it or not, the color of your home can actually make a huge difference when determining the ideal size of your air conditioner. Both color and material will impact how much heat is absorbed versus how much is reflected.

For example, dark colors and materials like brick will absorb heat. In this case, you may need a larger AC unit. Light colors will reflect heat, and materials like vinyl siding provide good insulation. In this case, you may be able to get away with a smaller unit.

Heat-Generating Appliances

Rooms with heat-generating appliances, like kitchens with stoves and refrigerators, will take more energy to cool down. If you’re looking for a window unit for a laundry room or kitchen, err on the side of a slightly larger one.

Signs You Have the Wrong Size Air Conditioning Unit

It can be hard to tell if your current air conditioner is the right size or not, especially if you weren’t the one who picked it. Here are common signs that the capacity of your current air conditioner isn’t right for your home.

• AC turns on and off all the time: Frequent cycling may point to an AC that’s too large for your home, creating inefficiencies that increase your energy bills.

• AC runs for a very long time: If your otherwise healthy AC system is running overtime, it’s likely too small for the home and struggling to cool it down in a timely manner.

• Uneven heating: If there are cold spots close to the thermostat but hot spots at the edges of your home, this is a classic sign of an oversized AC system cooling down the house too quickly and shutting off before it reaches the outer rooms.

• Poor cooling mixed with humid air: If the air in your home never dries out properly and there’s a mildew problem, your air conditioner may be too small.

Why Does HVAC Size Matter?

Think about this question: “What is HVAC?” Beyond its basic definition of heating, ventilation, and air conditioning, HVAC is a complex web of systems that all work together to make your life indoors as comfortable as possible. If parts of the system aren’t the right size to fit your needs, the whole unit will fail. A unit’s size is an important part of the entire system’s operation that can make or break its efficiency and have a huge impact on your home and your bank account.

Because of the price tag that’s attached to HVAC systems and components, some people are tempted to save money by opting for a smaller model, but doing that would definitely be a mistake. If your AC is too small, it will struggle to cool your home. This won’t just make you sweat; your air conditioner will have to work overtime, leading to premature wear and tear of many different system components and higher energy costs.

An AC that’s too large is also an issue. It cools too quickly, meaning it will turn on and off in short, repeated cycles. In other words, it’s still working overtime (but in a different way). This wears down your AC, raises your utility bills, and causes discomfort. You’ll have bursts of frigid air with periods of heat—not ideal.

A well-maintained unit that’s the right size will efficiently and evenly cool your home. It shouldn’t prematurely break down, and your home will be a lot more comfortable. You also won’t be wasting energy, which will reflect on your utility bill (and your wallet).

Understanding Air Conditioner Ratings 

There are a few different ratings that measure how your HVAC unit operates. The two most important measurable components of how well any given system works are the BTU rating, which measures the cooling capacity, and the SEER, which measures energy efficiency.

The Importance of BTUs 

What is a BTU? In short, it’s a measure of the amount of heat needed to raise the temperature by 1 degree Fahrenheit. BTU stands for British thermal unit, and there are 12,000 BTUs per one ton of AC capacity.

Air conditioning units remove BTUs from the atmosphere. For example, if you see an air conditioning unit rated for two tons, it can remove 24,000 BTUs from the air. More tonnage equals a higher rating, which indicates a larger cooling capacity. 

What Does SEER Stand For?

SEER (currently on the SEER 2 standard) stands for Seasonal Energy Efficiency Ratio (though you may also see the term Seasonal Energy Efficiency Rating). This rating, a whole number that can theoretically go up to 25, measures how efficiently your AC unit cools your home by weighing the cooling capacity with the total amount of electricity used. Higher ratings mean higher efficiency (and lower electric bills).

SEER2 ratings are required to be above a certain level, but that depends on the size of the AC system and the region where it’s installed. Most home AC is required to be 13-14 or higher. The Southwest and Southeast have higher SEER2 requirements than the colder northern states. The most efficient AC systems are usually several points higher than the requirements, but currently cap out in the low 20s.

Window Unit vs. Central AC Unit

If you’re only looking to cool one or two rooms, then window AC units or mini-split systems will suffice. If you need to cool the whole home, a central AC system will be a better investment in the long run, even though it has higher upfront costs. Installing a new central AC unit costs between $3,800 and $7,500 on average, but the maintenance costs and utility bills will be lower over the unit’s lifetime versus buying several window air conditioners. 

By now, you can probably answer the question, “What size AC unit do I need?” on your own with relative accuracy. However, it’s always safest to let a professional determine what size unit will suit you and your house the best. Call an air conditioner installer near you to have them visit your home and help you make the best decision possible. 

Amy Pawlukiewicz contributed to this piece.