BTU Calculator — Air Conditioner Sizing
Sizes an air conditioner for a room. Enter the floor area and pick the climate, and it gives the capacity needed in BTU, tons and kW, plus the nearest size actually sold.
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The formula
BTU/h = floor area (ft²) × climate factor × (ceiling height ÷ 8 ft) × sun × insulation + 600 per person over two + 4,000 if it is a kitchen
The climate factor is the whole calculation in one number: 20 BTU/h per square foot is the standard figure for a moderate summer, and hotter climates simply need more. Everything after it is a correction for the ways a real room differs from the average one the rule assumes.
What to watch out for
Three things move the answer more than people expect:
- Bigger is not better. An oversized unit satisfies the thermostat in a few minutes and shuts off, which is too soon to have pulled the humidity out. The room ends up cold and damp, and the compressor wears out from short-cycling.
- Sun matters more than area. A west-facing room with large windows can need 10–20% more than the same room facing north, because afternoon sun through glass is direct heat gain.
- This is a rule of thumb, not Manual J. A proper load calculation measures every wall, window and door and its U-value. For a single room and a plug-in or split unit, the rule of thumb is what installers use too. For a whole-house ducted system, pay for the survey.
Frequently Asked Questions
How many BTU do I need per square foot?
About 20 BTU/h per square foot in a moderate summer climate, rising to 25–30 in hot or humid ones and falling to around 16 where summers are mild. A 250 ft² room in a moderate climate therefore needs roughly 5,000 BTU/h before adjustments.
How many tons is 12,000 BTU?
Exactly one ton. A ton of refrigeration is defined as 12,000 BTU per hour — the rate at which melting one short ton of ice over 24 hours absorbs heat. So 24,000 BTU/h is 2 tons, and 36,000 is 3.
What happens if the air conditioner is too big?
It cools the air fast, hits the set temperature and stops before it has dehumidified. You get a cold, clammy room, more on-off cycling, higher running cost and shorter compressor life. Undersizing simply means the unit runs continuously on the hottest days, which is far less harmful.
Does ceiling height change the BTU needed?
Yes, roughly in proportion. The rule of thumb assumes an 8 ft ceiling; a 10 ft ceiling holds 25% more air and needs about 25% more capacity for the same floor area.