BTU Calculator
Size heating and cooling capacity from room area and ceiling height.
How this calculator works
BTU = area × BTU per sq ft × (ceiling height ÷ 8)
This is a rule-of-thumb estimate. A proper Manual J load calculation accounts for insulation, window area and orientation, air leakage and occupancy, and often lands 20 to 30% below the rule of thumb.
Worked example
With room area 1000 sq ft, ceiling height 8 ft, btu per square foot 25 , this calculator returns 25000 .
Frequently asked questions
How many BTUs do I need per square foot?
For cooling, 20 to 30 depending on climate. For heating, 30 in the deep south up to 60 in the far north. These are rules of thumb — a Manual J calculation is the proper method.
How many BTUs are in a ton of cooling?
Twelve thousand. A 3 ton air conditioner delivers 36,000 BTU per hour. The term dates from the cooling equivalent of melting a ton of ice over 24 hours.
Is it bad to oversize an air conditioner?
Yes. An oversized unit cools the air fast then shuts off before removing humidity, leaving the house cold and clammy. Short cycling also wears the compressor and wastes energy.
Does ceiling height affect BTU requirements?
Directly. The load scales with volume, not floor area. A room with 10 ft ceilings needs 25% more capacity than the same footprint at 8 ft.