🔥Furnace Size Calculator
Find a ballpark furnace size for your home in seconds — square footage, climate zone, insulation and efficiency.
Your home
Furnace input needed
94,737 BTU/hr
Heat output ≈ 90,000 BTU/hr · nearest common size 100,000 BTU input
output BTU
90,000
input BTU
94,737
BTU per sq ft
45
common size
100k
This is a rule-of-thumb estimate. A professional Manual J load calculation is the accurate way to size a furnace — oversizing wastes money and short-cycles.
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How it works
- Heat needed (output) = square feet × BTU per sq ft for your climate zone × insulation factor.
- Furnace input = output ÷ AFUE. A 95% furnace turns 95% of the gas it burns into heat.
Default BTU per sq ft: zone 1 = 30, 2 = 35, 3 = 40, 4 = 45, 5 = 50, 6 = 55, 7 = 60. Insulation: poor ×1.2, average ×1.0, good ×0.85.
Worked example
A 2,000 sq ft home in zone 4 with average insulation and a 95% furnace:
- 2,000 × 45 = 90,000 BTU/hr output
- 90,000 ÷ 0.95 = 94,737 BTU/hr input → a common 100,000 BTU furnace
Get it right
This is a rule of thumb. A Manual J load calculation by an HVAC pro accounts for windows, air leaks, orientation and duct losses and is the accurate method.
Questions people ask
Input is the fuel the furnace burns; output is the heat delivered. Output = input × AFUE.
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Last updated October 8, 2026. These results are friendly estimates to help you plan — always double-check before buying or making big decisions.