BTU Calculator
With room area 300 sq ft, ceiling height 8 ft, people in the room regularly 2, heating factor for your climate 40 BTU per sq ft, btu comes to 6,000 BTU/hr — cooling capacity needed. It is reached in 10 steps, the last of which is 6000 * 1 * 1 + 0 + 0, and each one is printed on the page with its numbers filled in. The formula is the one published by ENERGY STAR, not an approximation fitted to it.
The cooling or heating capacity a room needs, from its size and the things that change the answer — sun, ceiling height, occupants and a kitchen.
Formula and sources checked · How we check
Room area 300 sq ft, Ceiling height 8 ft, Sunlight Average, People in the room regularly 2
6,000 BTU/hr
Cooling capacity needed for the example below. Editing a field recomputes the calculator below; this figure holds the answer the page was loaded with.
It is written into the HTML rather than drawn by a script, so a search engine reading this page without running JavaScript still finds an answer.
- Base cooling load, 20 BTU per sq ft
300 * 206,000 BTU/hr- Adjustment for ceiling height
8 / 81- Sunlight adjustment
1- Extra for people beyond two
max(0, 2 - 2) * 6000 BTU/hr- Extra for a kitchen
0 BTU/hr- Cooling capacity needed
6000 * 1 * 1 + 0 + 06,000 BTU/hr- Nearest standard unit
ceil(6000 / 1000) * 10006,000 BTU/hr- In tons of cooling
6000 / 120000.5 tons- Heating capacity needed
300 * 40 * 112,000 BTU/hr- Cooling capacity in watts
6000 * 0.29307111,758.426 W
Several at once
A job is rarely one of anything. Work out the first, add it, change the figures and add the next — the list keeps what each line was, and totals them.
Worked example
A 300 square foot room with an ordinary aspect needs about 6,000 BTU an hour of cooling — half a ton. Buying 12,000 instead does not cool it better: it cools it faster, shuts off before the air has been dried, and leaves the room cold and clammy.
How to work it out yourself
- 1.Measure the floor area of the room being conditioned, not the whole floor. A unit sized for the house will short-cycle in a bedroom.
- 2.Adjust for what changes the load rather than the size: a sunny west-facing room, a high ceiling, a kitchen, or a room that regularly holds more than two people.
- 3.Round up to the nearest standard unit, but only just. Oversizing is a worse mistake than undersizing by a small margin.
Cooling capacity by room size
| Room area (sq ft) | Cooling capacity needed | Nearest standard unit | In tons of cooling |
|---|---|---|---|
| 100 | 2,000 BTU/hr | 2,000 BTU/hr | 0.17 tons |
| 150 | 3,000 BTU/hr | 3,000 BTU/hr | 0.25 tons |
| 200 | 4,000 BTU/hr | 4,000 BTU/hr | 0.33 tons |
| 250 | 5,000 BTU/hr | 5,000 BTU/hr | 0.42 tons |
| 300 | 6,000 BTU/hr | 6,000 BTU/hr | 0.5 tons |
| 400 | 8,000 BTU/hr | 8,000 BTU/hr | 0.67 tons |
| 500 | 10,000 BTU/hr | 10,000 BTU/hr | 0.83 tons |
| 600 | 12,000 BTU/hr | 12,000 BTU/hr | 1 tons |
| 700 | 14,000 BTU/hr | 14,000 BTU/hr | 1.17 tons |
| 1000 | 20,000 BTU/hr | 20,000 BTU/hr | 1.67 tons |
Average sunlight, 8 ft ceiling, two people, not a kitchen.
The formula
- Base cooling load, 20 BTU per sq ft
300 * 20 - Adjustment for ceiling height
8 / 8 - Sunlight adjustment
- Extra for people beyond two
max(0, 2 - 2) * 600 - Extra for a kitchen
- Cooling capacity needed
6000 * 1 * 1 + 0 + 0 - Nearest standard unit
ceil(6000 / 1000) * 1000 - In tons of cooling
6000 / 12000 - Heating capacity needed
300 * 40 * 1 - Cooling capacity in watts
6000 * 0.2930711
Source: ENERGY STAR — room air conditioners and sizing guidance, NIST Special Publication 811 — Guide for the Use of the International System of Units
Questions people actually ask
- What happens if the air conditioner is too big?
- It cools the room quickly and then switches off, which sounds like a good thing and is not. Removing humidity takes running time, and a short cycle never gets there — the room ends up cold and damp. An oversized unit also wears faster from starting and stopping, and it costs more to buy and to run.
- How many BTU per square foot?
- About 20 for cooling, as a base, before adjustments — that is the ENERGY STAR figure. For heating it depends on the climate: roughly 30 BTU per square foot in the warm south, 40 in the middle of the country and 50 to 60 in the far north. Insulation and window quality move both figures substantially.
- What is a "ton" of cooling?
- 12,000 BTU an hour, and it is a genuinely literal unit — the rate at which melting a ton of ice over 24 hours absorbs heat. A three-ton central system is 36,000 BTU an hour. Room units are quoted in BTU and central systems in tons, which is why the two never look comparable.
- Does this replace a proper load calculation?
- No. A rule-of-thumb figure sizes a window unit for one room. A whole-house system should be sized with a Manual J calculation, which accounts for insulation, window area and orientation, air leakage and local design temperatures — and it routinely comes out smaller than the rule of thumb suggests.
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