What Size Air Conditioner Do I Need? BTU Chart by Room Size

Air conditioner capacity is measured in BTU per hour, and the right number depends mainly on the size of the room. A 300 square foot room needs about 7,000 BTU, so you would shop for an 8,000 BTU unit. Choose too small and it runs all day without keeping up. Choose too big and it cools the room in a few minutes, switches off, and leaves the air damp and clammy.

BTU by room size

This table follows the room-size guidance published by Energy Star for room air conditioners:

Window and portable units come in standard sizes (5,000, 6,000, 8,000, 10,000, 12,000, 14,000 BTU and so on), so round up to the next one available.

Adjust for your room

The chart assumes a typical room. Real rooms differ, and a few adjustments get you closer:

• Heavily shaded room: reduce capacity by about 10%.
• Very sunny room: increase capacity by about 10%. West-facing windows with no shade are the worst offenders.
• More than two people: add about 600 BTU for each extra person who regularly uses the room.
• Kitchen: add about 4,000 BTU, since cooking generates a lot of heat.
• Tall ceilings: scale up in proportion to the extra height. A 10 ft ceiling means 25% more air to cool than an 8 ft one.
• Open to another space: if the room opens onto a hallway or another room without a door, include the area of both.

The AC size calculator applies all of these for you.

Why bigger is not better

An oversized air conditioner pulls the temperature down so fast that the compressor shuts off before it has run long enough to remove moisture. The result is a room that is cold but muggy, with the unit turning on and off constantly. Short cycling also wears the compressor, wastes power, and makes more noise. A correctly sized unit runs longer at a steady pace, which dehumidifies better and costs less to run.

Window, portable, or ductless mini-split?

• Window units are the cheapest and most efficient way to cool a single room. They need a suitable window and some way of sealing the gaps.
• Portable units sit on the floor and vent through a hose in a window. They are easy to move, but less efficient than a window unit of the same BTU rating, since the hose and the compressor inside the room both add heat. Look for a dual-hose model.
• Ductless mini-splits are the best choice for permanently cooling (and heating) a room or a whole floor. They are quiet and efficient, but cost more and need professional installation.
• Central air should be sized by a contractor using a proper heat load calculation (Manual J), not by a rule of thumb. Sizing is also in tons, where one ton is 12,000 BTU per hour.

Check the electrical requirements

Most window units up to about 12,000 BTU run on a standard 120 V outlet, but larger units need a dedicated 240 V circuit. Do not use an extension cord with an air conditioner, and do not share a circuit with other high-draw appliances. If you are not sure, ask an electrician before you buy.

Ways to need less cooling

• Close blinds or curtains on sunny windows during the day.
• Seal gaps around the window unit and the window frame.
• Run ceiling fans, which make the room feel several degrees cooler.
• Avoid heat-producing appliances like ovens and dryers during the hottest part of the day.
• Check that the air conditioner's filter is clean. A dirty filter can cut performance noticeably.