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Construction Calculators
Furnace Size Calculator
Estimate the furnace BTU output needed to heat a home based on square footage and climate zone.
Base heating load
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Required output
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Recommended furnace size
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About the Furnace Size Calculator
This calculator gives a simplified estimate of the furnace output needed to heat a home, using a BTU-per-square-foot rule of thumb that scales with your local climate severity.
Formula: Base Load = Square Footage × BTU Factor (30 warm, 45 moderate, 55 cold, 60 very cold); Required Output = Base Load × (1 + Duct Loss %)
This rule-of-thumb ignores insulation quality, ceiling height, window area and air infiltration — for an accurate furnace size, have an HVAC contractor perform a full Manual J heat loss calculation before purchasing.
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