BTU Calculator: Sizing Your AC or Heater
Published Apr 13, 2026 · 6 min read
An undersized unit won't cool/heat effectively. An oversized one cycles on and off too frequently, wasting energy. Correct BTU sizing is critical.
Basic BTU Formula
BTUs needed = Square footage Γ 20 (baseline for average insulation)
A 300 sq ft room needs about 6,000 BTU. But several factors adjust this number.
Adjustment Factors
| Factor | Adjustment |
|---|---|
| High ceiling (>8 ft) | +10-20% |
| Direct sunlight room | +10% |
| Heavily shaded room | -10% |
| Kitchen | +4,000 BTU |
| Each additional person (regularly) | +600 BTU |
| Poor insulation | +20-30% |
Common Room Sizes
| Room Size | BTU Range |
|---|---|
| 100-200 sq ft | 5,000-6,000 |
| 200-400 sq ft | 6,000-8,000 |
| 400-600 sq ft | 8,000-12,000 |
| 700-1,000 sq ft | 14,000-18,000 |
Try it: Use our BTU Calculator to get a precise recommendation based on your room's specifics.