Cooling · Room sizing
What size AC for a 400 sq ft room?
A 400 sq ft room with 8 ft ceilings needs about 45,307 BTU/h (3.8 tons) of cooling in a hot climate — buy a 48,000 BTU unit, or 60,000 BTU if the room gets direct sun.
By ceiling height
| Ceiling | BTU/h (shaded) | BTU/h (sunny) | Tons |
|---|---|---|---|
| 8 ft | 45,307 | 54,368 | 3.8 |
| 9 ft | 50,970 | 61,164 | 4.2 |
| 10 ft | 56,634 | 67,960 | 4.7 |
Hot-climate volumetric method (500–600 BTU/h per m³) — the same formula as our BTU calculator, where you can enter exact dimensions. In mild climates use the room-size chart's 20–25 BTU/sq ft column instead.
FAQ
- How many BTU for 400 sq ft?
- In a hot climate with 8 ft ceilings: about 45,307 BTU/h shaded, or 54,368 BTU/h with direct sun. In a mild climate the 20–25 BTU/sq ft rule gives 8,000–10,000 BTU/h — use that as the floor for insulated rooms.
- How many tons is that?
- 45,307 BTU/h equals 3.8 tons of cooling (1 ton = 12,000 BTU/h) — about 13.3 kW.