BTU Calculator
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About 7,000 BTU/h of cooling for 280 ft²; heating roughly 11,900 BTU/h
This sizes a single-room air conditioner and gives a rough heating figure. Sizing central air, a heat pump or a furnace for a whole house needs an ACCA Manual J load calculation.
How to use the BTU calculator
- Enter the room's length and width, or its floor area if you already know it, and the ceiling height.
- Choose the sun exposure, enter how many people regularly use the room and say whether it is a kitchen.
- Read the cooling capacity to look for in a room air conditioner, shown in BTU/h, kilowatts and tons.
- For heating, enter the temperature rise you need and keep or change the insulation factor to get a rough heater size.
Room air conditioner sizing formula
Heavily shaded: base × 0.9 · Very sunny: base × 1.1
+ 600 BTU/h for each regular occupant over two
+ 4,000 BTU/h if the unit is used in a kitchen
Heating (rule of thumb) ≈ room volume (ft³) × temperature rise (°F) × 0.133
1 kW = 3,412.1 BTU/h · 1 ton of cooling = 12,000 BTU/h
A BTU (British thermal unit) is the heat needed to warm one pound of water by 1 °F. Air conditioner capacity is a rate, BTU per hour, though labels often just say "BTU". The chart and adjustments come from ENERGY STAR's room air conditioner guidance, which assumes an 8 ft ceiling. ENERGY STAR also warns against oversizing: a unit that is too large cools the room before it has a chance to remove the humidity, and costs more to buy and run.
The heating line is much rougher. It multiplies the volume of air by the temperature rise and a factor of 0.133 BTU/h per cubic foot per °F (about 2.48 W per m³ per °C), a common rule of thumb for sizing a space heater for an average, moderately insulated room. It ignores windows, exterior walls and air leaks, so treat it as a starting point, not a heat-loss calculation.
Worked example: a sunny 14 × 20 ft bedroom
The room is 280 ft², which falls in the chart's 250 up to 300 ft² band: 7,000 BTU/h. It gets strong afternoon sun, so add 10% (+700), and three people use it, one more than the chart assumes (+600). That makes 8,300 BTU/h, about 2.43 kW. Look for a unit close to that rating. To heat the same room with 8 ft ceilings from 30 °F to 70 °F, the rule of thumb gives 2,240 × 40 × 0.133 ≈ 11,917 BTU/h, or about 3.49 kW.
Kitchens need the biggest bump. A 12 × 15 ft kitchen would need 6,000 BTU/h as a bedroom but 10,000 BTU/h as a kitchen, because cooking adds a lot of heat.
ENERGY STAR room air conditioner sizing chart
Capacity before adjustments, for 8 ft (2.4 m) ceilings. Each band includes its lower limit and runs up to, but not including, the upper one.
| Area to be cooled | Metric | Capacity needed | In kW |
|---|---|---|---|
| 100 up to 150 ft² | 9.3–13.9 m² | 5,000 BTU/h | 1.47 kW |
| 150 up to 250 ft² | 13.9–23.2 m² | 6,000 BTU/h | 1.76 kW |
| 250 up to 300 ft² | 23.2–27.9 m² | 7,000 BTU/h | 2.05 kW |
| 300 up to 350 ft² | 27.9–32.5 m² | 8,000 BTU/h | 2.34 kW |
| 350 up to 400 ft² | 32.5–37.2 m² | 9,000 BTU/h | 2.64 kW |
| 400 up to 450 ft² | 37.2–41.8 m² | 10,000 BTU/h | 2.93 kW |
| 450 up to 550 ft² | 41.8–51.1 m² | 12,000 BTU/h | 3.52 kW |
| 550 up to 700 ft² | 51.1–65 m² | 14,000 BTU/h | 4.1 kW |
| 700 up to 1,000 ft² | 65–92.9 m² | 18,000 BTU/h | 5.28 kW |
| 1,000 up to 1,200 ft² | 92.9–111.5 m² | 21,000 BTU/h | 6.15 kW |
| 1,200 up to 1,400 ft² | 111.5–130.1 m² | 23,000 BTU/h | 6.74 kW |
| 1,400 up to 1,500 ft² | 130.1–139.4 m² | 24,000 BTU/h | 7.03 kW |
| 1,500 up to 2,000 ft² | 139.4–185.8 m² | 30,000 BTU/h | 8.79 kW |
| 2,000 up to 2,500 ft² | 185.8–232.3 m² | 34,000 BTU/h | 9.96 kW |
Rough heating estimates by room size
8 ft ceilings and the 0.133 rule-of-thumb factor. A well-insulated room needs less; a drafty one or a room with large windows needs more.
| Room | Volume | 30 °F rise | 40 °F rise | 50 °F rise |
|---|---|---|---|---|
| 10 × 12 ft | 960 ft³ | 3,830 BTU/h | 5,107 BTU/h | 6,384 BTU/h |
| 12 × 14 ft | 1,344 ft³ | 5,363 BTU/h | 7,150 BTU/h | 8,938 BTU/h |
| 14 × 20 ft | 2,240 ft³ | 8,938 BTU/h | 11,917 BTU/h | 14,896 BTU/h |
| 20 × 20 ft | 3,200 ft³ | 12,768 BTU/h | 17,024 BTU/h | 21,280 BTU/h |
| 20 × 30 ft | 4,800 ft³ | 19,152 BTU/h | 25,536 BTU/h | 31,920 BTU/h |
Tips for choosing a room air conditioner
- Don't oversize. A bigger unit is not better. It short-cycles, leaves the room clammy and uses more electricity.
- Think about where it goes. In a corner or an odd-shaped room, pick a unit that can direct the airflow where you need it.
- Treat open-plan spaces as one area. If the room opens to a hallway or another room without a door, the air conditioner is cooling both.
- Whole-house systems are different. Central air, heat pumps and furnaces should be sized with an ACCA Manual J load calculation, which accounts for insulation, windows, orientation, air leakage and your local climate. Ask your HVAC contractor for one.
- Compare running costs with the electricity calculator, or measure the room with the square footage calculator.
Frequently asked questions
How many BTUs do I need for a 12 × 12 room?
A 12 × 12 ft room is 144 square feet, which the ENERGY STAR chart puts in the 100 up to 150 ft² band: 5,000 BTU per hour. Add 10% if the room is very sunny, take 10% off if it is heavily shaded, and add 600 BTU/h for each regular occupant over two.
What size air conditioner do I need for 500 square feet?
The ENERGY STAR chart gives 12,000 BTU/h for 450 up to 550 square feet, about 3.5 kW or one ton of cooling. A sunny room needs about 13,200 BTU/h, and a kitchen of that size needs 16,000 BTU/h.
How do sun, shade, people and kitchens change the BTUs?
ENERGY STAR recommends reducing capacity by 10% for a heavily shaded room, increasing it by 10% for a very sunny room, adding 600 BTU/h for each person over two who regularly uses the room, and adding 4,000 BTU/h if the unit is used in a kitchen.
How many BTUs are in a ton or a kilowatt?
One ton of cooling is 12,000 BTU per hour, and one kilowatt is about 3,412 BTU per hour. So an 8,000 BTU/h air conditioner provides about 2.3 kW of cooling, and a 24,000 BTU/h unit is a 2-ton unit.
Can I use this to size central air or a furnace?
No. The chart is for single-room air conditioners, and the heating figure is a rough rule of thumb. Whole-house heating and cooling equipment should be sized with an ACCA Manual J load calculation, which accounts for insulation, windows, air leakage and your climate.