HVAC / BTU Calculator
Explore approximate heating and cooling loads with clearly labeled rule-of-thumb assumptions.
Preparing your calculator…
How this calculator works
Base heating uses 40 BTU/hr per sq ft; cooling uses 20. Both scale with ceiling height / 8 and insulation factor (0.8, 1, 1.25). Climate factors are heating (0.75, 1, 1.35), cooling (1.2, 1, 0.85); sun factors are cooling (0.9, 1, 1.15). Heating adds 100 per window; cooling adds 200 per window and 600 per occupant above two. Cooling tons = BTU/hr ÷ 12,000. This omits envelope, infiltration, glazing area, duct losses, and design weather calculations.
Use current quotes, local rates, and measured dimensions to make this estimate useful for your situation.
Frequently asked questions
Can I size equipment from this number?+
No. Rule-of-thumb loads can be substantially wrong for a specific building. Obtain a Manual J calculation and equipment selection from a qualified professional.
Why do insulation and sun change results?+
They influence heat transfer and solar gains. The factors here are broad planning assumptions, not measured envelope performance.