How many BTU per square foot?
A common rule of thumb is about 20 BTU per square foot, but the Energy Star room air conditioner chart is more precise for small rooms. It assumes 8 ft ceilings and two occupants, then adjusts for sun, extra people and kitchens.
| 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 |
| 1,000–1,200 ft² | 21,000 BTU |
| 1,200–1,400 ft² | 23,000 BTU |
| 1,400–1,500 ft² | 24,000 BTU |
| 1,500–2,000 ft² | 30,000 BTU |
| 2,000–2,500 ft² | 34,000 BTU |
Adjustments
- Heavily shaded room: −10%. Very sunny room: +10%.
- More than two people: +600 BTU per extra person.
- Kitchen: +4,000 BTU.
- Ceilings above 8 ft and poor insulation or a hot top floor raise the load; this calculator scales by ceiling height and uses ±10–20% for insulation as a rule of thumb.
Worked example
A 300 ft² sunny bedroom used by three people: chart value 7,000 BTU × 1.10 for sun = 7,700 BTU, plus 600 BTU for the third person = 8,300 BTU. Round up to 9,000 BTU (about 2.6 kW or 0.75 ton).
Frequently asked questions
How many BTU do I need for a 12x12 room?
A 12×12 room is 144 ft², so the Energy Star chart gives 5,000 BTU. Add 10% if it is very sunny.
Is a bigger air conditioner better?
No. An oversized unit cools the air quickly and shuts off before it removes humidity, so the room feels cold and clammy and the compressor wears faster.
How many square feet does 1 ton of AC cool?
One ton is 12,000 BTU/h, which the Energy Star chart matches to 450–550 ft² for a single room.