How to estimate BTUs
The simplest sizing rule is based on floor area:
BTU per hour = room area (sq ft) × BTU per sq ft
About 20 BTU per square foot is a common baseline for cooling. Divide the result by 12,000 to express it in tons of air conditioning (1 ton = 12,000 BTU/hr).
Adjust for your room
| Condition | Adjustment |
|---|---|
| Very sunny room | add ~10% |
| Heavily shaded room | subtract ~10% |
| Kitchen | add ~4,000 BTU |
| Each person over two | add ~600 BTU |
Good to know
- Don't oversize. Too-big units short-cycle, wasting energy and leaving the air humid.
- Ceiling height matters. The area rule assumes ~8 ft ceilings; tall rooms need more.
- Heating varies by climate. Cold regions need a higher BTU-per-sq-ft figure than the cooling baseline.