A 400 sq ft room (37.2 m²) typically needs 14,000 BTU of cooling capacity — a commonly sold air conditioner size that covers the average case.
Sun exposure, insulation and floor level all shift the number. For a 400 sq ft room:
| Typical room (average conditions) | 12,100 BTU |
| Shaded, well-insulated room | 7,300 BTU |
| Sunny, poorly insulated room | 14,400 BTU |
These are calculated loads; the actual unit you'd buy is rounded up to the nearest common size (14,000 BTU for the typical case above).
Get an exact recommendation for your room →These are computed with ACtopus's room-sizing formula (the base rule of thumb is about 20 BTU per square foot, adjusted for ceiling height, sun, insulation and floor level) — the same calculation the interactive calculator uses. Read the full breakdown in Cooling Sizing & BTU Load.