What Size Air Conditioner Do I Need?

Getting a properly sized air conditioning system is a smart move. One that is too small for your space will leave you warm, or it will burn out quickly because it has to work too hard to maintain the temperatures you want. On the other hand, one that is too large will cost too much when you purchase and will probably use more power than necessary while its running. You need to find that middle ground that works best. A professional like Air With Care (airwithcare.com.au) can be very helpful if you have questions but you should be able to work out some rough estimates on your own.

Air Conditioning System in Room

Air Conditioning System in Room

For this article, we are mainly talking about window or single-room units. Central air conditioning systems are more complex to measure on your own and you might want to talk to an HVAC specialist in those cases.

What is Size?

When you’re talking about air conditioners, “size” usually means the power rather than the actual dimensions of the unit. The unit for a/c power is the BTU, and most units will range between 5,000 to more than 30,000 BTUs. This is the “size” of an air conditioner.

Square Footage

Now that you understand the size idea, you need to see how much space you are going to be cooling. The average room is some sort of square or rectangular shape, so you just need to measure the length and width. Multiply those two numbers together and you have the square footage. Oddly shaped rooms will just take a little more math, but you can usually get a rough estimate working with a rectangular formula.

Size of Air Conditioning System

Size of Air Conditioning System

If you need two separate rooms cooled, you can’t really just add their areas together. The walls and door will block the air-flow enough that only one room will benefit from the cooling, even if you size the model to the overall area.

Once you have your area, here is a quick reference chart for air conditioner sizes:

Size of Area Between   Required A/C Power
150 and 250 sq ft   6,000 BTUs
250 to 350 sq ft   7,500 BTUs
350 to 450 sq ft   9,500 BTUs
450 to 550 sq ft   12,000 BTUs
550 to 700 sq ft   14,000 BTUs
700 to 1,000 sq ft   18,000 BTUs
1,000 to 1,200 sq ft   21,000 BTUs
1,200 to 1,400 sq ft   23,000 BTUs

If your cooling needs include several rooms and individual room units are impractical, you will want a central unit that provides cool air through your existing ducts rather than window units. As we mentioned, that’s when a specialist comes in.

Air Conditioner Size

Air Conditioner Size


You also need to consider the temperatures you usually deal with. Are you living in an area that often reaches 95°F or higher in the summers, or are is your local climate more moderate? Not everyone bothers to work this into their sizing calculations but you will be smart to take your location into account. For areas with very hot summers, add about 10% in BTUs to any estimates you make based on room area.

One last tip” don’t try to undersize your air conditioner in order to save on power costs. An efficient unit that has the EnergyStar certification will help on costs instead. Get the proper size.