How It Works
A full Manual J load calculation scores every surface of your home. This calculator captures the five biggest factors in a simplified, transparent formula:
- Floor area — 22 BTU/ft² cooling base, 24–38 BTU/ft² heating base by climate.
- Ceiling height — +10% per foot above 8 ft.
- Windows — +1,000 BTU cooling, +600 BTU heating per window.
- Sun exposure — ±10% on the cooling load.
- Occupants — +600 BTU cooling / +400 BTU heating per person beyond two.
Calculation Explanation
Example: a 1,000 sq ft four-season home, 8-ft ceilings, 6 windows, normal sun, 3 people:
Cooling = (1,000 × 22) + 6,000 (windows) + 600 (extra person) = 28,600 BTU ≈ 2.5 tons.
Heating = (1,000 × 30) + 3,600 (windows) + 400 = 34,000 BTU ≈ 3 tons.
Since heating often exceeds cooling in four-season climates, a heat pump or furnace should be sized to the larger number. Compare with the BTU Calculator for single rooms, or the AC Tonnage Calculator for a pure cooling-tonnage answer.
Heating BTU by Climate (per sq ft)
| Climate | Heating BTU/ft² | Cooling BTU/ft² | Example Region |
|---|---|---|---|
| Cold | 38 | 22 | Minnesota, Maine |
| Four seasons | 30 | 22 | Ohio, Virginia |
| Hot | 24 | 22 | Florida, Texas |
| Mild | 26 | 22 | California coast |