AC Sizing

Buying a Room Air Conditioner

When buying a room air conditioner, remember that bigger is not always better. If you were to buy a unit that is too large, you aren’t necessarily going to get better cooling. An oversized air conditioner is actually less effective - it also wastes energy as well. Make sure your unit is sized properly.

So What is a Proper Size?

Air conditioners remove both heat and humidity from the air, and if a unit is too large, it will cool the room in less time, but will only remove some of the humidity. This will leave you with a damp, clammy feeling room - if properly sized, the unit will remove humidity as effectively as it cools.

There is a simple formula to size a unit best for your needs:

Determine the square footage of the area to be cooled - use the following.

  • For rectangular (or square) rooms - multiply the length of the area by the width.
  • For a triangular space, multiply the length of the area by the width, and divide by 2.

For odd shape rooms, break down the room into basic shapes, and apply the same math as above.

Use the chart below can help you to determine the correct cooling capacity for your room. Cooling capacity is measured in British Thermal Units (BTU’s) per hour.

Area To Be Cooled (square feet)

Capacity Needed (BTUs per hour)

100 up to 150


150 up to 250


250 up to 300


300 up to 350


350 up to 400


400 up to 450


450 up to 550


550 up to 700


700 up to 1,000


1,000 up to 1,200


1,200 up to 1,400


1,400 up to 1,500


1,500 up to 2,000


2,000 up to 2,500


Make any adjustments for the following circumstances:

  • If the room is heavily shaded, reduce capacity by 10 percent.
  • If the room is very sunny, increase capacity by 10 percent.
  • If more than two people regularly occupy the room, add 600 BTUs for each additional person.
  • If the unit is used in a kitchen, increase capacity by 4,000 BTUs.
  • Consider where you install the unit. If you are mounting an air conditioner near the corner of a room, look for a unit that can send the airflow in the right direction.

As always, do your research and make sure you’re taking all the factors necessary into consideration before making your purchase. In this case, choosing the wrong size will be counter-productive, end up costing you more in the long run, and leave you with a less than cool alternative to just opening the windows and using your ceiling fan.

Stop by Reno’s Appliance in Paterson, NJ to select the best air conditioning unit.