BTU Calculator
Find the right air conditioner size in BTU for a room based on square footage, sun exposure, occupants and kitchen use.
Sizing an air conditioner
Cooling capacity is measured in British thermal units per hour (BTU/h). The ENERGY STAR sizing chart, used by the US Department of Energy for room air conditioners, starts from floor area:
| Room area | Capacity |
|---|---|
| 100–150 ft² | 5,000 BTU |
| 150–250 ft² | 6,000 BTU |
| 250–300 ft² | 7,000 BTU |
| 300–350 ft² | 8,000 BTU |
| 350–400 ft² | 9,000 BTU |
| 400–450 ft² | 10,000 BTU |
| 450–550 ft² | 12,000 BTU |
| 550–700 ft² | 14,000 BTU |
| 700–1,000 ft² | 18,000 BTU |
Adjustments
- Heavily shaded room: reduce capacity by 10%.
- Very sunny room: increase by 10%.
- More than two regular occupants: add 600 BTU per extra person.
- Kitchen: add 4,000 BTU.
Why not just buy bigger?
An oversized unit cools the air quickly and shuts off before removing humidity, leaving the room cold and clammy, and it cycles more often, wasting energy. An undersized unit runs constantly and never reaches the set temperature. For central air in a whole house, a contractor's Manual J load calculation is the proper method.
Frequently asked questions
How many BTU do I need for a 12x12 room?
A 144 ft² room needs about 5,000 BTU, or 6,000 BTU if it is very sunny.
How many square feet does 12,000 BTU cool?
About 450–550 square feet in average conditions.
What is a ton in AC terms?
One ton of cooling is 12,000 BTU per hour.
Last reviewed October 2026. Results are estimates; check important decisions with a qualified professional.