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BTU Calculator Guide
Cooling and heating capacity for a room, from area and the handful of things that genuinely change the answer.
Base cooling ≈ 20 BTU per square foot at an 8ft ceiling.
A 400 sq ft room ≈ 8,000 BTU, then adjusted for sun, ceiling height, occupancy, insulation and whether it's a kitchen.
12,000 BTU = 1 ton.
Bigger Is Not Better — It's Worse
This is the one that matters, and it's counterintuitive enough that people get it wrong on purpose.
An oversized air conditioner cools the room fast, hits the thermostat setpoint, and shuts off. That sounds ideal. But removing humidity takes time — the coil has to run long enough for moisture to condense on it. A unit that short-cycles leaves you with a room that's cold and clammy, and short-cycling wears the compressor out faster too. A correctly sized unit runs longer, gently, and actually dries the air.
Heating Is Climate, Not a Constant
Cooling load is reasonably predictable because indoor and outdoor temperatures differ by a fairly consistent amount almost anywhere. Heating isn't.
A mild climate might need 30 BTU per square foot; a severe one can need 60 or more. That's a 2× spread on the same room, which is why this page gives you a range rather than a single confident number. Your local climate zone is the input that decides it.
What This Doesn't Know About Your House
A rule of thumb per square foot is a starting point, not a design. It can't see your window area, orientation, air leakage, duct losses, roof colour, or what's above and below the room.
For a window unit or a single mini-split, this is good enough. For a whole-house system, a proper load calculation — the industry standard is a Manual J — is what you should be paying for, and any contractor who sizes a system off square footage alone is guessing with your money.
Ceiling Height Is Volume, Not Area
The 20 BTU/sq ft baseline assumes an 8ft ceiling. A room with 12ft ceilings has 50% more air in it and needs proportionally more.
This calculator scales for that. It's a common miss in older houses and converted spaces, where the floor area looks modest and the volume isn't.
People and Appliances Are Heaters
A human body puts out heat continuously. Beyond the first two occupants, add roughly 600 BTU each — which matters in a home office with four people or a dining room that hosts.
Kitchens get a flat addition because the appliances dump heat directly into the room. An oven running is a substantial heat source that no amount of insulation helps with.
Related: ceiling fan size calculator, electricity bill calculator, square footage calculator.
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