Rule-of-thumb BTU per sq ft by climate zone
These BTU-per-square-foot bands are rule-of-thumb estimates by IECC climate zone, not a load calculation. They give a first-pass size; a professional Manual J accounts for insulation, windows, orientation and infiltration. Use the AC-size and furnace-size tools to apply them with adjustments.
Cooling — rule-of-thumb BTU/sq ft
| Climate | BTU/sq ft | 1,500 sq ft ≈ |
|---|---|---|
| Cool / northern | 20 | 30,000 BTU (2.5 tons) |
| Moderate / mixed | 25 | 37,500 BTU (3.1 tons) |
| Warm / southern | 30 | 45,000 BTU (3.8 tons) |
| Hot / desert | 35 | 52,500 BTU (4.4 tons) |
Heating — rule-of-thumb BTU/sq ft
| Climate | BTU/sq ft | 1,500 sq ft ≈ |
|---|---|---|
| Mild winter | 30 | 45,000 BTU output |
| Moderate | 40 | 60,000 BTU output |
| Cold | 50 | 75,000 BTU output |
| Very cold | 60 | 90,000 BTU output |
Rule-of-thumb estimates by IECC climate zone — a starting point, not a Manual J load calculation.