What a BTU measures
A British thermal unit (BTU) is the energy needed to raise one pound of water by one degree Fahrenheit. Air conditioners and heaters are rated in BTU per hour (BTU/h): how much heat they can remove from or add to a room each hour. A 10,000 BTU window AC removes 10,000 BTU of heat per hour when running at full output.
Cooling and heating needs are different numbers. A room loses heat to the outside in winter faster than it gains heat in summer in most of the U.S., so heating BTU is usually higher than cooling BTU, especially in cold climates.
BTU per square foot by climate
Starting values for a space with 8 ft ceilings, average insulation and average sun. Heating values include the safety margin normally used to size heaters and furnaces.
| Climate zone | Cooling BTU/sq ft | Heating BTU/sq ft | Example cities |
|---|---|---|---|
| Hot · IECC 1–2 | 26.7 | 30 | Miami, Houston, Phoenix |
| Warm · IECC 3 | 22.9 | 35 | Atlanta, Dallas, Los Angeles |
| Mixed · IECC 4 | 20.9 | 40 | Nashville, St. Louis, Baltimore |
| Cold · IECC 5 | 19.2 | 50 | Chicago, Denver, Boston |
| Very cold · IECC 6–7 | 18.5 | 60 | Minneapolis, Burlington, Duluth |
BTU by room size
Mixed climate (IECC zone 4), 8 ft ceilings, two people, average insulation and sun.
| Room area | Example | Cooling BTU/h | Heating BTU/h |
|---|---|---|---|
| 100 sq ft | 10 × 10 ft | 2,100 | 4,000 |
| 150 sq ft | 10 × 15 ft | 3,100 | 6,000 |
| 200 sq ft | 10 × 20 ft | 4,200 | 8,000 |
| 250 sq ft | 12.5 × 20 ft | 5,200 | 10,000 |
| 300 sq ft | 15 × 20 ft | 6,300 | 12,000 |
| 400 sq ft | 20 × 20 ft | 8,300 | 16,000 |
| 500 sq ft | 20 × 25 ft | 10,400 | 20,000 |
| 700 sq ft | 25 × 28 ft | 14,600 | 28,000 |
| 1,000 sq ft | 25 × 40 ft | 20,900 | 40,000 |
Window air conditioners come in 5,000, 6,000, 8,000, 10,000, 12,000, 15,000, 18,000 and 24,000 BTU sizes. Mini split heads come in 6,000, 9,000, 12,000, 18,000 and 24,000 BTU. Choose the nearest size at or above your result.
BTU to watts, kW and tons
One watt equals 3.412 BTU per hour. One ton of cooling equals 12,000 BTU per hour, or 3.517 kW.
| BTU/h | Watts | kW | Tons |
|---|---|---|---|
| 5,000 | 1,465 | 1.47 | 0.42 |
| 6,000 | 1,758 | 1.76 | 0.50 |
| 8,000 | 2,345 | 2.34 | 0.67 |
| 10,000 | 2,931 | 2.93 | 0.83 |
| 12,000 | 3,517 | 3.52 | 1.00 |
| 18,000 | 5,275 | 5.28 | 1.50 |
| 24,000 | 7,034 | 7.03 | 2.00 |
| 36,000 | 10,551 | 10.55 | 3.00 |
| 48,000 | 14,068 | 14.07 | 4.00 |
| 60,000 | 17,585 | 17.58 | 5.00 |
A 1,500-watt plug-in space heater puts out about 5,100 BTU/h, enough to take the chill off a 120 sq ft room in a mixed climate but not to heat it as the only source.
Adjustments the calculator makes
- Ceiling height. Values scale with ceiling height above or below 8 ft.
- Insulation. Poor insulation adds 15% to cooling and 25% to heating; good insulation removes 10% and 15%.
- Sun. Shade lowers cooling by 10%; strong sun raises it by 10%. Sun does not change heating needs, which are set by the coldest nights.
- People. Each person beyond two adds 600 BTU/h of cooling.
- Room type. Kitchens add 4,000 BTU/h of cooling for cooking heat. Sunrooms add 30% to both. Garages add 25% to cooling and 50% to heating because they are rarely insulated. Basements need 20% less cooling and 10% less heating, as the ground moderates temperature.
Frequently asked questions
How many BTU do I need per square foot?
For cooling, about 19 to 27 BTU per square foot depending on climate, with 20 BTU a common average. For heating, about 30 BTU per square foot in hot climates up to 60 BTU in very cold climates.
How many BTU to heat 1,000 sq ft?
About 30,000 BTU/h in a hot climate, 40,000 BTU/h in a mixed climate and 60,000 BTU/h in a very cold climate, assuming 8 ft ceilings and average insulation.
How many BTU to cool 500 sq ft?
About 10,400 BTU/h in a mixed climate and 13,300 BTU/h in a hot climate. A 12,000 BTU window unit or mini split covers it in most of the U.S.
How many BTU do I need to heat a two-car garage?
A typical two-car garage is about 400 to 500 sq ft. Uninsulated, it needs about 24,000 to 30,000 BTU/h in a mixed climate, or roughly 7 to 9 kW of electric heat.
How do I convert BTU to kW?
Divide BTU per hour by 3,412. For example, 12,000 BTU/h ÷ 3,412 = 3.52 kW.
How many BTU is a ton?
One ton of cooling is 12,000 BTU per hour. A 3-ton central AC delivers 36,000 BTU/h.
These figures are estimates for planning. Equipment for a whole home should be sized with a Manual J load calculation by a licensed contractor.