BTU calculator
A BTU per hour is the unit that air conditioners, heat pumps and furnaces are sized in. Fill in the room to size a window or wall air conditioner, or fill in the house for a first estimate of central cooling and heating.
- Room, 300 sq ft8,000 BTU/h
- House cooling, rule of thumb48,000 to 60,000 BTU/h
- House cooling, efficient house24,000 to 30,000 BTU/h
- House heating80,000 to 100,000 BTU/h
Room figure: the ENERGY STAR chart for room air conditioners, less 10% for heavy shade, plus 10% for strong sun, plus 600 BTU/h for each person beyond two and 4,000 BTU/h for a kitchen.
House cooling: the common estimate is one ton, 12,000 BTU/h, for every 400 to 500 square feet. A very efficient house needs one ton for every 800 to 1,000 square feet. Most houses fall between the two.
House heating: 40 to 50 BTU/h of furnace output per square foot, a furnace manufacturer’s rule of thumb for this climate.
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BTU needed for one room
Room air conditioners are sized by the floor area of the room. This is the ENERGY STAR sizing chart. It assumes an 8 foot ceiling.
| Room floor area (sq ft) | Capacity needed (BTU/h) |
|---|---|
| 100 up to 150 | 5,000 |
| 150 up to 250 | 6,000 |
| 250 up to 300 | 7,000 |
| 300 up to 350 | 8,000 |
| 350 up to 400 | 9,000 |
| 400 up to 450 | 10,000 |
| 450 up to 550 | 12,000 |
| 550 up to 700 | 14,000 |
| 700 up to 1,000 | 18,000 |
| 1,000 up to 1,200 | 21,000 |
| 1,200 up to 1,400 | 23,000 |
| 1,400 up to 1,500 | 24,000 |
| 1,500 up to 2,000 | 30,000 |
| 2,000 up to 2,500 | 34,000 |
“Up to” means just under. A room of exactly 150 square feet belongs to the next row.
BTU needed for a whole house
No government agency or standards body publishes BTU per square foot by climate for a whole house. These tables combine two US Department of Energy (DOE) cooling figures with a furnace maker’s heating rule of thumb.
| Cooling capacity (BTU/h) | ||
|---|---|---|
| Floor area (sq ft) | Common estimate | Very efficient house |
| 1,000 | 24,000 to 30,000 | 12,000 to 15,000 |
| 1,500 | 36,000 to 45,000 | 18,000 to 23,000 |
| 2,000 | 48,000 to 60,000 | 24,000 to 30,000 |
| 2,500 | 60,000 to 75,000 | 30,000 to 38,000 |
| 3,000 | 72,000 to 90,000 | 36,000 to 45,000 |
Rounded to the nearest 1,000. Common estimate: one ton per 400 to 500 square feet. Very efficient house: one ton per 800 to 1,000 square feet.
| Furnace output (BTU/h) | |||
|---|---|---|---|
| Floor area (sq ft) | warm climate | moderate climate | cold climate |
| 1,000 | 30,000 to 35,000 | 40,000 to 50,000 | 50,000 to 60,000 |
| 1,500 | 45,000 to 53,000 | 60,000 to 75,000 | 75,000 to 90,000 |
| 2,000 | 60,000 to 70,000 | 80,000 to 100,000 | 100,000 to 120,000 |
| 2,500 | 75,000 to 88,000 | 100,000 to 125,000 | 125,000 to 150,000 |
| 3,000 | 90,000 to 105,000 | 120,000 to 150,000 | 150,000 to 180,000 |
Rounded to the nearest 1,000.
How the calculator works
- Room: a published chart. The calculator starts from the ENERGY STAR chart figure for the floor area. It subtracts 10% for a heavily shaded room and adds 10% for a very sunny one. It adds 600 BTU/h for each person beyond two and 4,000 BTU/h for a kitchen.
- House cooling: government figures. One ton of cooling is 12,000 BTU/h. The US Department of Energy’s Consumer Guide to Home Heating and Cooling gives both cooling figures. DOE says the common estimate ignores climate and the efficiency of the house. It says contractors should use ACCA Manual J.
- House heating: a rule of thumb. Bryant, a furnace maker, gives 30 to 35 BTU/h per square foot for warm climates. It gives 40 to 50 for moderate and 50 to 60 for cold. Warm covers the South and Southwest, moderate the Midwest and Mid-Atlantic, and cold the Northeast and northern Midwest. Bryant tells readers to get a load calculation from a dealer.
Limits of this calculator
- Floor area is not a load. The load is the BTU/h your specific house needs. ACCA Manual J, the residential load calculation, finds it. The cooling ranges do not change with climate.
- Oversizing has a cost. ENERGY STAR warns that an oversized room air conditioner cools the room before it removes humidity.
- Taller ceilings need a higher rating. ENERGY STAR gives that advice.
- The room chart stops just under 2,500 square feet. Use the whole-house figures for a larger room.
Sources
- US Department of Energy, Consumer Guide to Home Heating and Cooling: https://www.energy.gov/sites/default/files/2021-08/ES-HomeHeatingandCooling_081221.pdf
- ENERGY STAR, Room Air Conditioners: https://www.energystar.gov/products/room_air_conditioners
- Bryant, furnace sizing page: https://www.bryant.com/en/us/products/gas-furnaces/what-size-furnace-do-i-need/
- ACCA, Manual J, residential load calculation: https://www.acca.org/standards/technical-manuals/manual-j
Related
For a furnace rating, use the furnace size calculator. For central cooling in tons, use the AC tonnage calculator. For a ductless system, use the mini-split sizing calculator. For the figures behind the rules, read how many BTU per square foot.
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Calculator last reviewed on October 5, 2026. Method and sources.