❄️ BTU Calculator
An air conditioner that is too small will not cool the room; one that is too big cycles on and off and leaves air damp. Enter the room size to find the right capacity.
How the BTU Calculator works
The base capacity comes from the ENERGY STAR room air conditioner sizing chart. It is reduced 10% for heavily shaded rooms, increased 10% for very sunny rooms, increased 600 BTU per person above two, and increased 4,000 BTU for kitchens.
Step-by-step calculation
- 1Enter room size.
- 2Find the base BTU from the ENERGY STAR chart.
- 3Adjust for sun, people and kitchen use.
Worked example
A 12 × 20 ft bedroom (240 ft²) with normal sun and 2 people needs about 6,000 BTU.
BTU Calculator examples
| Room length (ft) | Recommended AC size |
|---|---|
| 6 | 5,000 BTU/h |
| 9 | 6,000 BTU/h |
| 12 | 6,000 BTU/h |
| 15 | 7,000 BTU/h |
| 18 | 9,000 BTU/h |
Other inputs kept at the calculator's default values. Results calculated by this calculator.
Why use a BTU Calculator?
Choose an air conditioner that cools without wasting energy.
Important factors that can affect your results
- Insulation and windows.
- Ceiling height.
- Climate.
Frequently asked questions
What does BTU mean?
British Thermal Unit — the heat needed to raise one pound of water by 1 °F. AC capacity is listed in BTU per hour.
What about whole-house systems?
Central air needs a proper load calculation (Manual J) by an HVAC professional.
Related calculators
Last reviewed November 2026 · How we build and check our tools · Report a problem