Furnace Size Calculator

Furnace BTU = ft² × climate-zone factor (30-60 BTU/ft²).

Inputs

AFUE: Annual Fuel Utilization Efficiency. 80% standard, 90-98% high-efficiency.

Result

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General calculation reads

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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 vs output BTU?+
Input = energy consumed. Output = heat delivered. Output = input × AFUE. The label rating is usually input.
80% vs 90% AFUE?+
80% is older standard (atmospheric vent); 90-95% is condensing furnace (PVC vent). 90%+ saves 10%+ on gas bills.
Is bigger always better?+
No — oversized furnaces short-cycle, leaving cold spots and wear. Right-size for your home.
When to use 65 BTU/ft²?+
Extreme cold climates (MN, ND, Alaska). Old construction in cold areas may need 70-80.
Heat pump alternative?+
Heat pumps for moderate climates; dual-fuel (heat pump + gas furnace) for cold. Gas furnaces dominate in cold climates still.

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 (sqft OR BTU input)
1 ton = 12,000 BTU/hr cooling. Convert sqft → tons (rule-of-thumb sizing) OR BTU → tons (direct).
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.
kWh Cost Per Month
Cost = W × hr/day × 30 / 1000 × $/kWh.

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