Furnace Heating Load Calculator
Estimate a room or home heating load in BTU/hr from area, ceiling height, climate and insulation, then size the furnace input for its AFUE.
How it works
Load = area × climate factor × (ceiling ÷ 8) × insulation factor · Furnace input = load ÷ (AFUE ÷ 100)
Heating load scales with floor area and the temperature difference your climate demands, expressed here as BTU/hr per square foot. Insulation and air-tightness swing the number widely — a leaky older home can need 25% more heat than a tight new build of the same size. A furnace is rated by its input; its usable output is that input times the AFUE, so a 95% furnace must be fed slightly more fuel energy than the load it delivers.
Worked example
A 1,500 ft² moderate-climate home with 8 ft ceilings and average insulation: 1,500 × 40 × 1.0 = 60,000 BTU/hr of load. A 95% AFUE furnace needs 60,000 ÷ 0.95 ≈ 63,200 BTU/hr of input.
Frequently asked questions
Does higher AFUE mean a bigger furnace?
No — a higher AFUE means less fuel is wasted, so a high-efficiency furnace needs a slightly smaller input rating to deliver the same heat. The output (the heating load) is what must match your home.
Why not just buy the biggest furnace available?
An oversized furnace short-cycles, heating the house in brief bursts that cause uneven temperatures, more wear, and lower comfort. Sizing near the calculated load gives longer, steadier, more efficient runs.
Is this the same as a Manual J calculation?
It is a quick estimate. A full Manual J load calculation accounts for each wall and window’s insulation, orientation, infiltration and your exact design temperature, and is what a contractor should use before installation.
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This calculator is for educational purposes. Double-check important results before acting on them.