Furnace Size Calculator
Furnace BTU = ft² × climate-zone factor (30-60 BTU/ft²).
Result
Furnace input rating
90,000 BTU/hr
81,000 BTU/hr heat output (90% AFUE).
- Area1800 ft²
- Climate factor45 BTU/ft²
- Heat output needed81,000 BTU/hr
- AFUE90%
- Furnace input rating90,000 BTU/hr
Step-by-step
- Heat output = 1800 × 45 = 81,000 BTU/hr.
- Furnace input = output / AFUE = 81,000 / 0.9 = 90,000 BTU/hr.
How to use this calculator
- Enter heated area.
- Pick climate zone.
- Adjust AFUE based on furnace efficiency.
About this calculator
Furnaces are rated by INPUT BTU (gas burned). Output BTU = input × AFUE (efficiency). Modern high-efficiency furnaces are 90-98% AFUE; older standard 80%. Standard residential furnaces: 60k, 80k, 100k, 120k BTU input — pick the one that meets or slightly exceeds the calculated need.
Frequently asked
Input = energy consumed. Output = heat delivered. Output = input × AFUE. The label rating is usually input.
Related calculators
BTU for Room Heating Calculator
Heating BTU/hr = ft² × insulation factor × ΔT.
BTU for Room Cooling Calculator
BTU/hr needed = ft² × climate factor (20-30 BTU/ft² for typical US climates).
AC Tonnage Calculator
1 ton = 12,000 BTU/hr cooling. Quick estimate: ft²/600 → tons.
Heat Load Calculator (U-value method)
Heat loss = U × area × ΔT for each surface, summed.
Ductwork CFM Calculator
CFM (cubic feet per minute) = BTU/hr × 0.07. Used for sizing ducts.
Concrete Volume Calculator
Calculate cubic yards and cubic feet of concrete needed for a slab, footing, or pad — given length, width, and depth.