Home Improvement Skills & Specialties HVAC

What Size Air Conditioner Do I Need?

window unit air conditioner

The Spruce / Letícia Almeida 

When people speak of size in reference to air conditioners, it doesn't refer to the physical dimensions of the appliance itself. Rather, it means the cooling capacity is measured in BTUs (British Thermal Units), which is how much energy an air conditioning unit requires to cool the indoor air by removing heat. For example, 8,000 BTU means that a unit will use that much energy to remove heat and cool the indoor air in an hour.

Whether you are choosing a window air conditioner or a central air unit, you will get the best efficiency, and therefore lower operating costs, if you choose based on the size of the area the unit must cool.

To choose what size air conditioner is right for you, you need to first calculate the size of the home spaces served by the air conditioner, in square feet.

Determining Air Conditioner Size

To calculate the size, simply multiply the length times the width of the room or area to be cooled. Then, as a practical number, multiply that total times 25 BTU. This allows ample cooling, whether it is a rainy, moist day or a hot, sunny, humid day. Let’s say the room is 12 feet wide by 15 feet long. That means 12x15=180 square feet. Take the 180 sq. ft. times 25 BTU per square foot and you get the minimum BTU air conditioner you should buy. That means 180x25=4500 BTU cooling capacity is needed.

You may ask yourself: will a smaller air conditioner work or will a larger one be better? Here’s a thought about those questions. Smaller BTU units will continually run, just to try to keep up. This will increase your electric bill and it is unlikely the unit will be able to cool the area effectively.

Air Conditioner Sizes

 Square Footage BTUs AC Size in Tons
600-1,000 square feet 18,000 1.5
1,000-1,500 square feet 24,000  2
1,500-2,000 square feet 30,000 2.5
2,000-2,500 square feet 34,000 3
2,500-3,300 square feet 42,000 3.5

Factors That Affect Air Conditioner Sizing

While looking at the recommended size for an air conditioning unit can be helpful, there are factors that can impact the formula.

  • Ceiling height: Typically, recommended BTUs are calculated on a standard 9-foot ceiling height. If you have higher ceilings, there will be more energy required to cool the space.
  • Climate: Because warmer climates will require running AC throughout more of the year, you'll need to look for a unit with higher Seasonal Energy Efficiency Ratio (SEER).
  • Insulation: A well-insulated house will do a better job of cooling a home efficiently since it will keep the cool air in.
  • Windows: Windows also need to be well-insulated in order to keep the cool air in. Double- or triple-pane windows are best.
  • Sun exposure: If your home faces south or west and gets great natural light, that's usually a good thing — unless you're trying to cool a house. Homes that are shadier will cool more efficiently.

Why Does AC Size Matter?

Purchasing the right size AC unit for your home is critical. If your system is not big enough for your space, it won't be able to adequately cool your space, and you'll spend money on a system that doesn't get you the intended result.

If your AC unit is too big, then it also won't be able to efficiently cool your space. It will cool your home too quickly, then shut off, then turn on again, which can cause higher bills because of the constant on and off.

Choosing the Right AC Unit for Your Home

Beyond BTUs, there are also various types of AC units that you can choose for your home. Central air conditioning is what most people think of when they hear air conditioning. It's a whole house system. However, in homes where there are spots that stay warm or if you aren't able to invest in central AC, then window units or mini-split systems are a great alternative.

Window units run from $150 to $800, and will efficiently cool one room at a time. A mini-split system can be added to central AC to target specific rooms and areas. These will run from $600 to $2,000 per unit.

When comparing units, consider the cost, size required, and your home's own factors that impact the efficiency of an AC unit.

FAQ
  • How do I calculate what size air conditioner I need?

    To calculate the right size air conditioner, look at your square footage and compare it to the size BTUs that space will need. Also consider other factors, like climate, insulation, sun exposure, and ceiling height.

  • How many square feet does a 3 ton AC unit cover?

    A 3 ton AC unit should cover 2,000-2,500 square feet.

  • What sizes do air conditioners come in?

    For a residential home, you can typically purchase 18,000-60,000 BTU air conditioning units, which is 1.5 to 5 tons.