How to Size Your Air Conditioner

Air conditioners on apartment building at sundown
Jaan Bernberg/Moment Open/Getty Images

The more powerful your air conditioner, the better-right? Actually, no.

Not only is buying a higher capacity air conditioner more expensive upfront, it can unnecessarily run up your energy bills during the summer.

Even worse, it can leave your house feeling clammy. A too-small AC can be a problem too; it works too hard to cool the space and you still end up with high electrical bills. An air conditioner that is, as Goldilocks would say, "just the right size" not only cools the space, but it removes humidity at the same time.

Learn how to size your air conditioner for your entire house or a single room.

Doing the Math

Air conditioners' cooling capacity is measured in British Thermal Units, or BTUs. You may also hear AC size referred to in "tons," which is the equivalent of 12,000 BTU's. While it's not necessary to understand the science behind it, you should have a good grasp of capacity in relationship to the square footage you need to cool. Simple math skills will suffice.

First, find out the area of your room by multiplying the length by the width. For oddly-shaped rooms, reach back to basic principles you learned in geometry class. Once you have determined the entire square footage of your home or of the room you plan to cool, multiply it by 25 BTUs. The resulting number is the BTU capacity required. For example:

A 15 foot wide by 20 foot long room contains 300 square feet. Multiply this by 25 BTU, and you'll discover the room requires a 6,000 BTU capacity unit.

Keep in mind that this is a very rough estimate. It's pretty adequate for sizing a room AC, but other factors can also affect the size of AC you need for a particular space. Keep these factors in mind:

  • For a heavily shaded room, reduce the BTU capacity by 10%; if it is sunny, increase it by 10%
  • If more than two people occupy the room regularly, add an additional 600 BTU per each extra person
  • Kitchens produce a lot of heat, so add 4,000 BTU to your total

Sizing a Central AC

When you purchase a new central AC, you will most likely consult an HVAC specialist or a large home improvement store that handles installations. Professionals typically use the scientifically accurate "Manual J" method to determine the right size. Still, it's a good idea to be familiar with the factors involved.

The size of your home is obviously one of the largest factors in determining the right AC size, but it's not the only one. You must also consider your home's orientation (relative to the sun's); how well it is shaded; the number and size of windows as well as which directions they face; and how well your walls and air ducts are insulated. Play around with an online calculator to get a general idea of how all these factors come together:


The electric bills you receive during the summer months are probably the highest of the year, thanks to your air conditioner. If yours is on its last leg or you've just moved into a new apartment and need a window unit, learn how to calculate the proper size. A properly sized AC rewards you with energy savings all summer long.

EnergyStar's air conditioner sizing tutorial

Gartner HVAC