FURNACE SIZE
FURNACE SIZE
BTU/hr = ft² × climate_factor
ft²
RESULT
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FILL IN ABOVE
Rule of thumb based on climate zone. AFUE 80% common for older furnaces, 90-96% for high-efficiency condensing models. Input BTU = output BTU ÷ AFUE.
About this calculator
This furnace size calculator returns the heating output a residential furnace needs for a given conditioned area and climate zone. The rule of thumb runs 25-60 BTU per square foot of heating output depending on climate severity, building tightness, and insulation level. The calculator returns both heating output BTU/hr (what the equipment delivers to the house) and input BTU/hr (the rated nameplate size, accounting for typical 80-95% AFUE efficiency). For tight specs in cold climates or large houses, an HVAC contractor should run an ACCA Manual J load calculation against the actual building envelope.