BTU Calculator

How to size a room air conditioner

An air conditioner that is too small never cools the room; one that is too large cools so fast it cycles off before it can remove humidity, leaving the space cold and clammy. Sizing is about matching the unit's capacity — measured in BTU per hour — to the heat load of your room.

The starting point is floor area. The widely used rule of thumb is 20 BTU per hour for every square foot of floor space, assuming a standard ceiling about 2.44 m. A 3.66 × 4.27 m room is 15.6 m², which works out to a base load of about 3,360 BTU/hr before any adjustments.

The adjustments that matter

• Ceiling height: taller ceilings mean more air to cool. We scale the base load by ceiling height ÷ 2.44 m.
• Sun exposure: a heavily shaded room needs about 10% less capacity; a very sunny room about 10% more.
• Occupants: two people are already covered by the base load; add 600 BTU/hr for each additional person who is regularly in the room.
• Kitchens: cooking appliances throw off a lot of heat, so a kitchen adds roughly 4,000 BTU/hr.

Regional notes

In the humid South and along the Gulf Coast, prioritise a unit that dehumidifies well and avoid oversizing — an oversized unit short-cycles and leaves the air damp. In the dry Southwest, cooling is more about raw heat than humidity, so sizing at or slightly above the calculated load is fine. In either case, add capacity for a top-floor room under a hot roof.

Frequently asked questions

Is it better to buy a bigger BTU unit to be safe? No. An oversized air conditioner cools the room quickly, then shuts off before it has pulled enough moisture from the air, so the room feels cold and clammy and the compressor wears from short-cycling. Size to the load, not above it.

Does this work for mini-splits and portable units too? The BTU load of the room is the same regardless of unit type, so the target capacity here applies. Portable units lose some effective capacity to their exhaust hose, so choose one rated at or slightly above the recommended figure.

What if my room opens into another room? Open-plan spaces share their cooling load. Add the square footage of any space that stays open to the room and size for the combined area.

These figures are planning estimates based on published rule-of-thumb guidance. For a whole-home central system or an unusual room (lots of glass, cathedral ceilings, a converted garage), a professional Manual J load calculation is the accurate approach. See our heating and cooling guides for more.