BTU calculator
How many BTUs of air conditioning a room needs, from the ENERGY STAR sizing chart, with tons and watts.
Cooling needed
6,000 BTU per hour
- Tons of cooling
- 0.50
- In watts of cooling
- 1,758 W
- Chart size for 168 sq ft
- 6,000 BTU/h
- The ENERGY STAR chart stops at 2,500 sq ft. A space this large, or a whole house, needs a Manual J load calculation from an HVAC contractor.
From ENERGY STAR's room air conditioner sizing chart, adjusted for sun, people beyond 2 and kitchens as ENERGY STAR describes. "Watts of cooling" is the cooling capacity, not the power the unit draws; check the label for that. A Manual J calculation is the accurate answer for central air, heat pumps and whole houses.
How many BTUs do I need?
For a room air conditioner, ENERGY STAR publishes a sizing chart by room area, plus adjustments for sun, people and kitchens. The calculator looks up your room on the chart and applies them.
- Area = length × width, in square feet
- Chart size: the BTU/h for the area band your room falls in (table below)
- Sun: −10% for a heavily shaded room, +10% for a very sunny one
- People: +600 BTU/h for each person beyond 2 who regularly uses the room
- Kitchen: +4,000 BTU/h
- Tons = BTU/h ÷ 12,000; watts = BTU/h × 0.2930711
Worked example
A 16 × 20 ft living room is 320 sq ft, which the chart sizes at 8,000 BTU/h. It's very sunny, so add 10%: 8,800. 4 people use it, 2 more than 2, so add 1,200 for 10,000 BTU/h, or 0.83 tons. Look for a unit at or just above that size.
Air conditioner BTU chart by room size
From ENERGY STAR, Room Air Conditioners sizing chart, before the sun, people and kitchen adjustments.
| Room area | BTU per hour | Tons |
|---|---|---|
| 100 up to 150 sq ft | 5,000 | 0.42 |
| 150 up to 250 sq ft | 6,000 | 0.50 |
| 250 up to 300 sq ft | 7,000 | 0.58 |
| 300 up to 350 sq ft | 8,000 | 0.67 |
| 350 up to 400 sq ft | 9,000 | 0.75 |
| 400 up to 450 sq ft | 10,000 | 0.83 |
| 450 up to 550 sq ft | 12,000 | 1.00 |
| 550 up to 700 sq ft | 14,000 | 1.17 |
| 700 up to 1,000 sq ft | 18,000 | 1.50 |
| 1,000 up to 1,200 sq ft | 21,000 | 1.75 |
| 1,200 up to 1,400 sq ft | 23,000 | 1.92 |
| 1,400 up to 1,500 sq ft | 24,000 | 2.00 |
| 1,500 up to 2,000 sq ft | 30,000 | 2.50 |
| 2,000 up to 2,500 sq ft | 34,000 | 2.83 |
Watts, tons and BTU per hour
Conversion factors from NIST SP 811, Appendix B.9: 1 BTU/h = 0.2930711 W, and a ton of cooling is 12,000 BTU/h.
| Watts | BTU per hour |
|---|---|
| 500 W | 1,706 |
| 1,000 W | 3,412 |
| 1,500 W | 5,118 |
| 2,000 W | 6,824 |
| 5,000 W | 17,061 |
| Tons | BTU per hour | Watts of cooling |
|---|---|---|
| 0.5 | 6,000 | 1,758 |
| 1.0 | 12,000 | 3,517 |
| 1.5 | 18,000 | 5,275 |
| 2.0 | 24,000 | 7,034 |
| 3.0 | 36,000 | 10,551 |
| 4.0 | 48,000 | 14,067 |
| 5.0 | 60,000 | 17,584 |
BTU questions
What does BTU stand for?
British thermal unit, a unit of heat energy equal to about 1,055 joules (NIST SP 811, Appendix B.9). Air conditioners and heaters are rated in BTU per hour, the rate at which they move heat, though labels often just say "BTU". A 10,000 BTU air conditioner removes 10,000 BTU of heat from the room every hour it runs at full power.
How many BTU per square foot do I need?
On the ENERGY STAR chart it varies with the room size rather than following one rate: a 400 sq ft room is sized at 10,000 BTU/h, 500 sq ft at 12,000 BTU/h and 1,000 sq ft at 21,000 BTU/h. That works out to fewer BTUs per square foot as rooms get bigger. Sun, people and a kitchen move it up or down, so use the calculator for your room.
How many BTU are in a ton?
A ton of cooling is 12,000 BTU per hour, about 3,517 watts of cooling. Central air conditioners and heat pumps are usually sold in tons, so a 3-ton unit moves 36,000 BTU/h.
How many BTU do I need to heat a room or a house?
It depends on your climate, insulation, windows and air leaks far more than on floor area, so this calculator doesn't give a heating figure from square footage. Room charts like the one above are for cooling. For a furnace, boiler or heat pump, ask an HVAC contractor for a Manual J load calculation, the room-by-room method used to size heating and cooling equipment. For a single space heater, the heater's label gives its output in BTU/h or watts; the conversion table above turns one into the other.
What size mini split or air conditioner do I need for a garage?
Mini splits are rated in BTU/h the same way, and the chart gives a starting point for a single room. Garages, attics and sunrooms usually gain more heat than the rooms the chart was made for, from uninsulated walls, big doors or lots of glass, so treat the result as a minimum and have the installer confirm the size.
How do I convert watts to BTU?
Divide watts by 0.2930711, or multiply by about 3.412: a 1,500 W space heater puts out about 5,118 BTU/h. To go from BTU/h to watts, multiply by 0.2930711. An air conditioner's cooling capacity in watts isn't the electricity it uses; its label lists the power draw separately.