What Size Air Conditioner Do I Need? How to Find the Right Unit for Your Home

Keep your home cool without wasting energy with our guide to air conditioner sizes.

air conditioner hvac outside brick wall

Getty Images / Thelma Lanteigne / EyeEm

Depending on the location, an air conditioner may be a temporary appliance only used for a few months of the year or a permanent part of your home's HVAC system used year round. These appliances work to decrease the temperature inside the home, so residents can sleep, eat, and work in a comfortable environment.

However, if an air conditioner is too small for the home, it will run constantly to try to keep up with demand, leading to premature wear and tear on the appliance. Despite running overtime, the air conditioner will likely be unable to meet the needs of the home, so it will never feel properly cooled.

Investing in an oversized air conditioner might cool the home, but a unit that is too large will waste a lot of energy, driving up heating and cooling bills. Instead, take some time to learn how to find the right unit for your home with the help of this informative guide, so you can answer the question: what size air condition do I need?

Understanding Air Conditioner Sizes

When trying to find the right air conditioner, it's important to have a basic understanding of air conditioner sizes. This does not refer to the physical size of the unit. Rather, air conditioner size is a measurement of the cooling capacity, which is typically represented in BTU.

BTU (British Thermal Unit) is used to measure energy. Specifically, it is a measurement of the amount of energy required to heat a pound of water by one degree Fahrenheit at sea level. When used with air conditioners, the higher the BTU rating, the greater the energy output of the appliance. This means an air conditioner with a high BTU rating will cool a home better than an air conditioner with a low BTU rating.

In some cases, the output of the air conditioner may be stated with a BTUh (British Thermal Units per hour) rating. This is essentially a measurement of how much energy the air conditioner uses to remove heat from the home within one hour.

However, this rating can also be stated in tonnage, with 12,000 BTUh equal to one ton. These terms are not as common, but it's still important to understand what a ton is and that there is a difference between BTUh and BTU.

Air Conditioner Size Considerations

Before getting into the necessary calculations to find the right air conditioner size, it's important to learn about the various factors that can affect the heating and cooling demands of a home. If you purchase an air conditioner without considering these variables, the unit you purchase might not be able to adequately cool the home.

  • Climate and location can affect the temperature of the home. Warmer locations typically require an air conditioner with a higher output than homes in colder climates.
  • Home decor can also increase or decrease the ambient temperature of a home, depending on color and material choices. Darker colors tend to absorb heat from the sun, while lighter colors reflect a portion of the heat to help keep the home cool.
  • Shade and sun exposure throughout the day can affect the size of air conditioner that is required to adequately cool the home. A home that is south- or west-facing generally takes longer to cool because it receives more sun exposure than a north- or east-facing home. However, homes with a lot of shade may require an air conditioner with a lower output because they tend to remain cooler throughout the day.
  • Insulation is designed to keep heat in during the winter and prevent heat from entering during the summer. An air conditioner in a well-insulated home won't need to work as hard as an air conditioner in a home with old, thin, or low-quality insulation.
  • Windows can be a problem if they are not properly sealed. This is because heat can seep in through the gaps, making the air conditioner work overtime. If drafty windows are an issue, consider sealing the windows or investing in an air conditioner with a higher energy output.
  • Ceiling height is a factor that is often overlooked. Most air conditioner size calculations are made with the assumption that the home has standard nine-foot ceilings. If your home has lower ceilings, invest in a smaller air conditioner size; if your home has high ceilings, you will need a larger air conditioner to cool the space.
  • Heat-generating appliances, like stovetops, ovens, and dryers, increase a room's temperature when in use. If you're looking for an air conditioner for your kitchen or laundry room, select a larger air conditioner that can handle the increased demand created by heat-generating appliances.
Window air conditioning unit installed in window
bgwalker / Getty Images

How to Calculate the Right Air Conditioner Size

In order to find an air conditioner that's the right size for your home, it's necessary to determine the square footage of the home, consider any factors that could affect the ambient temperature, and calculate the ideal BTU output.

Start by measuring the square footage. If you're only purchasing an air conditioner for a single room, you only need to measure that room. For whole-home air conditioners, it's necessary to measure the square footage for each room, then add the totals together to find the square footage for the entire home.

  • Rectangular Rooms: Multiply length by width to find the square footage.
  • Triangular Rooms: Multiply length by width, then divide that number by two to find the square footage.
  • Circular Rooms: Measure the distance from the center of the room to the edge. This is known as the radius. Calculate the radius squared, then multiply the result by 3.14 to find the square footage.
  • Odd-Shaped Rooms: Divide the room into pieces that make up regular shapes, then use the above methods to calculate square, rectangular, triangular, or circular parts of the room. Add the separate totals together to find the square footage of the room.

An air conditioner generally needs about 20 BTU per square foot of living space. Once you've calculated the square footage of the room or home, multiply the result by 20 to find the air conditioner size for the home. For example, if the square footage of the home is 2,000 square feet, you can find the right air conditioner size by multiplying 2,000 by 20. The result is 40,000 BTUs.

However, it's necessary to consider any factors that could affect the ambient temperature when determining the right size air conditioner for the home. Factors like living in a hotter climate, having a home that faces the sun for the majority of the day, or a dark home exterior can make it harder to keep a home cool, so you may need to increase the size of the air conditioner by a few thousand BTUs.

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  1. Room air conditioners. (n.d.). Energy.Gov. https://www.energy.gov/energysaver/room-air-conditioners

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