Furnace Sizing Calculator

Estimate the furnace input BTU a home needs from floor area, climate zone, and AFUE efficiency.

Inputs
sq ft
Results update as you type — no submit
ResultsLIVE
Furnace input size
66,316 BTU/hr
≈ 19.44 kW
Heat output needed63,000 BTU/hr · 18.46 kW
Nearest furnace80,000 BTU/hr input
Rule-of-thumb estimate — a Manual J load calculation by a licensed HVAC professional is definitive before you buy equipment.
Formula
output = 1,500 × 42 = 63,000 BTU/hr; input = ÷ 95% = 66,316 BTU/hr
How it works

A furnace is rated two ways. Output (or delivered) BTU is the heat that actually reaches your rooms; input BTU is the fuel it burns to make that heat. The nameplate size you shop for is the input number, and the two are linked by efficiency: input = output ÷ AFUE.

Sizing starts with the heat a house loses on a cold day, estimated as floor area times a climate-zone factor. Warm southern zones need only about `32` BTU/hr per square foot; the middle of the country runs near `42`; cold northern zones reach `48` to `55`. Multiply your heated area by the factor for your zone to get the required heat output. A `1,500` square-foot home in a mid-US Zone 3 needs about `63,000` BTU/hr of output.

Then divide by AFUE — the Annual Fuel Utilization Efficiency printed on the yellow EnergyGuide label — to find the input size to buy. At `95%` AFUE, `63,000` BTU/hr of output needs `66,316` BTU/hr of input, so you would shop the next stocked size, an `80,000` BTU furnace. These climate factors are planning rules of thumb; a Manual J load calculation that accounts for your insulation, windows, and air-sealing is the definitive sizing.

Worked example
Home `1,500` sq ft · Zone 3 (`42` BTU/ft²) · `95%` AFUE
output = 1,500 × 42 = `63,000 BTU/hr`
input = 63,000 ÷ 0.95 = `66,316 BTU/hr` → nearest furnace `80,000 BTU/hr`
Questions about furnace sizing
Multiply your heated square footage by a climate factor — about 32 BTU/hr per square foot in the Deep South up to 55 in the far north — then divide by the furnace's AFUE to get the input size. A 2,000 sq ft home in a cold Zone 4 needs roughly 96,000 BTU/hr of output, or about 120,000 BTU/hr input at 80% AFUE.
SOURCES — Published US climate-zone heating factors · US DOE / FTC AFUE definition
Rule-of-thumb estimate — a Manual J load calculation by a licensed HVAC pro is definitive
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