What size air conditioner do I need?

Last reviewed 2026-06-06

Too small and it never cools; too big and the room turns clammy. Here is how to size an AC by BTU — the right way.

Air conditioners are rated in BTU per hour — the amount of heat they can remove. Match the BTU to the room and it cools efficiently and quietly; get it wrong in either direction and you waste money and comfort. Here is how to size one properly.

Start with the area

The baseline rule of thumb is about 20 BTU per square foot of floor area. So a 300 sq ft room needs roughly 6,000 BTU before adjustments. Measure length × width to get your area.

Then adjust for the room

• Sun: add about 10% for a very sunny or west-facing room; subtract ~10% for a heavily shaded one.
• Ceilings: add for ceilings above the standard 8 ft — there is more air to cool.
• People: add roughly 600 BTU for each person beyond two who regularly use the room.
• Kitchen: add about 4,000 BTU — cooking throws off a lot of heat.

Why bigger is not better

An oversized unit cools the air fast then shuts off before it has pulled out the humidity, leaving the room cold and clammy — and the constant short-cycling wears the unit out and wastes energy. Slightly under and steady beats oversized and stop-start.

Common sizes

Window and portable units typically come in 5,000, 6,000, 8,000, 10,000 and 12,000 BTU steps. Pick the size at or just above your calculated need rather than jumping to the biggest one available.

Get your number

The air conditioner BTU calculator takes your room size and the conditions (sun, people, kitchen) and returns the BTU you need plus the unit size to buy — in both BTU and kilowatts.

Calculators for this

• Air Conditioner BTU Calculator — What size (BTU) air conditioner a room needs.
• Drywall Calculator — Sheets of drywall for walls and ceilings.