BTU Calculator
Size a heater, air conditioner or mini-split for a room from its dimensions, ceiling height, insulation, climate and sun exposure — in BTU per hour.
How many BTU do I need for this room?
A BTU (British Thermal Unit) is a unit of heating or cooling power — equipment is rated in BTU per hour. Sizing starts from the room's floor area, then adjusts for the things that make one room harder to heat or cool than another the same size: how tall the ceiling is, how well the home is insulated and sealed, how hot the climate gets, and how much sun the windows let in.
BTU/hr ≈ floor area (sq ft) × 25 × (ceiling height ÷ 8 ft) × insulation × climate × sun
Example: a 12 × 12 ft room with 8 ft ceilings, average insulation, a moderate climate and average sun needs 144 × 25 = ≈ 3,600 BTU/hr — a 5,000 BTU unit covers it, since that is the smallest standard size. Make the insulation poor and the room very sunny, and the same room needs 3,600 × 1.25 × 1.15 = ≈ 5,175 BTU/hr, so a 6,000 BTU unit is the honest pick.
Why the adjustments are multipliers
Each factor here is a multiplier, not a guess at a different formula. Poor insulation lets heat move through the walls about a quarter faster, so the equipment must supply roughly a quarter more. Tall ceilings add air volume in direct proportion — a 10 ft ceiling holds 25% more air than an 8 ft one, so the height enters as a simple ratio. Climate and sun work the same way: more heat arriving means more capacity to remove it.
| Room (8 ft ceiling, average everything) | Base need | Standard unit sizes |
|---|
| 10 × 10 ft (100 sq ft) | ≈ 2,500 BTU/hr | 5,000 BTU |
| 12 × 12 ft (144 sq ft) | ≈ 3,600 BTU/hr | 5,000 BTU |
| 15 × 15 ft (225 sq ft) | ≈ 5,625 BTU/hr | 6,000–8,000 BTU |
| 20 × 15 ft (300 sq ft) | ≈ 7,500 BTU/hr | 8,000–9,000 BTU |
| 20 × 20 ft (400 sq ft) | ≈ 10,000 BTU/hr | 10,000–12,000 BTU |
This is a sizing tool — it gives a capacity recommendation, so there is nothing to buy by volume and no waste allowance. For a whole-house system or a cold-climate heat pump, a professional load calculation (Manual J in the US) is the step after this estimate — but for a window unit, a portable or a single mini-split head, this is the math those purchases are made on. Kitchen heat, several occupants and big electronics all push the number up; add roughly 600 BTU per extra regular occupant in a small room.
Frequently asked questions
What happens if I buy a unit that is too big?
Counter-intuitively, it cools worse. An oversized air conditioner blasts the room to temperature and shuts off before it has run long enough to pull the humidity out, leaving the room cold and clammy — and it cycles on and off all day, which wears it and wastes power. Size to the calculation, not to “bigger must be better.”
Does this work for heating too?
For mild and moderate climates, yes — the same capacity figure is a reasonable planning number for a heat pump or electric heater. In cold climates, pure heating demand on the coldest nights can run well above the cooling-led figure; treat the result as a minimum there and have a heating contractor confirm before you commit.
How many BTU is a typical window air conditioner?
Small units are 5,000–6,000 BTU (one bedroom), mid sizes 8,000–12,000 (large bedroom or small living room), and large window or through-wall units reach 18,000–24,000. Mini-split heads commonly come in 6,000 / 9,000 / 12,000 / 18,000 / 24,000 BTU steps.
Why does ceiling height matter so much?
Equipment heats and cools air, and the room's air volume is area × height. A 12 ft ceiling holds half again as much air as an 8 ft ceiling in the same footprint — the calculator scales the need by height ÷ 8, so vaulted rooms are not quietly under-served.
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