Estimated cooling capacity you need
How to size an air conditioner (BTU explained)
BTU (British Thermal Units per hour) measures how much heat an air conditioner can remove — the bigger the room, the more BTU you need. As a rule of thumb the U.S. Department of Energy suggests about 20 BTU per square foot of living space, then you adjust for sun, ceiling height, how many people use the room, and whether it’s a hot kitchen. This calculator does that for you and points you to portable AC models in the right range.
Undersizing means the unit runs non-stop and never cools; oversizing cools too fast, leaves the room clammy, and wastes electricity. Getting the BTU right is the single most important choice when buying an AC.
This is a planning estimate based on standard U.S. DOE guidance. Very hot climates, poor insulation, or lots of windows may need a little more capacity — when in doubt, size up one step.
Frequently Asked Questions
How many BTU do I need for a bedroom?
Most bedrooms are 120–300 sq ft, which needs roughly 5,000–8,000 BTU. Enter your room’s exact size above for a tailored number — a sunny top-floor bedroom needs more than a shaded one.
Can a portable AC run all night?
Yes. A correctly sized unit can run continuously; look for a sleep mode and a timer so it eases off once the room is cool. A right-sized AC uses far less power overnight than an oversized one that short-cycles.
Why add BTU for a kitchen or sunny room?
Ovens, fridges and direct sunlight add heat the AC must also remove. Standard guidance adds about 4,000 BTU for a kitchen and roughly 10% for a very sunny room — both are built into this calculator.
Portable vs window AC — which should I get?
Portable units need no permanent install and move room to room (great for renters); window units are cheaper per BTU but must fit a window. See our buying guide for the best current picks in your size range.