BTU Calculator

British Thermal Unit is the heat needed to raise one pound of water by one degree Fahrenheit. In HVAC, BTU per hour (BTU/h) is how cooling and heating capacity is rated. A 12,000 BTU/h air conditioner removes 12,000 BTU of heat from a room each hour; a 30,000 BTU/h furnace adds that much heat.

The classic rule of thumb is roughly 20 BTU/h per square foot for cooling and around 30 BTU/h per square foot for heating, assuming 8-foot ceilings, average insulation, average sun, and a single occupant. The calculator scales by your actual ceiling height, insulation level, sun exposure, occupants, and whether the room is a kitchen.

No. ACCA Manual J is the industry-standard residential load calculation, and it accounts for individual wall constructions, window U-values, infiltration rates, duct losses, and climate zone. Permitted central-HVAC sizing requires Manual J or equivalent. This calculator is a quick first-pass sizing estimate, useful for room AC and space heater sizing.

One ton of cooling equals 12,000 BTU/h. So 3 tons of cooling capacity is 36,000 BTU/h. Central air conditioners are usually sized in tons; window units and portable ACs are usually sized in BTU/h.

An oversized air conditioner short-cycles: it cools the air fast but turns off before it pulls humidity out, leaving the space cold but clammy. Undersized AC runs constantly on hot days and never reaches the setpoint. Right-sized AC runs long enough to dehumidify but cycles off on cooler days.

Cooking appliances release significant heat. The calculator adds about 4,000 BTU/h of cooling load for kitchens (and reduces heating load slightly, since the appliances supplement the heating system).

The BTU rating system is most common in the U.S. and Canada. Other regions often use watts or kilowatts (1 kW ≈ 3,412 BTU/h). Pick a unit your local equipment uses; the underlying load is the same physical quantity.