Furnace Size Calculator — BTU & AFUE

    Estimate what size furnace your home needs based on square footage, location, insulation, and efficiency. Results in BTU/h and standard furnace sizes. Free — no signup required.

    sq ft

    How to Use

    1. Select your nearest city — this determines your climate zone and design temperature.
    2. Enter your heated square footage.
    3. Rate your insulation quality and window type — these adjust your heat load significantly.
    4. Select your ceiling height and the furnace AFUE you plan to buy.
    5. Click Calculate to get your recommended furnace size.

    Quick Reference — Standard Furnace Sizes

    Estimated furnace size by home size and climate. Assumes average insulation and 96% AFUE.

    Home SizeMildModerateColdVery Cold
    800 sq ft40k60k60k80k
    1,000 sq ft40k60k80k80k
    1,200 sq ft40k60k80k100k
    1,500 sq ft60k60k80k100k
    1,800 sq ft60k80k100k120k
    2,000 sq ft60k80k100k120k
    2,500 sq ft80k100k120k150k
    3,000 sq ft80k100k120k150k

    Example Calculation

    A 1,800 sq ft home in Ottawa, ON (design temp -25°C, moderate-to-cold zone, 30 BTU/sf heating factor) with average insulation (×1.0), double-pane windows (×1.0), 8-ft ceilings (×1.0), and a 96% AFUE furnace: load = 1,800 × 30 × 1.0 × 1.0 × 1.0 = 54,000 BTU/h. Input BTU = 54,000 / 0.96 = 56,250 BTU/h. Rounded up to the next standard size: 60,000 BTU furnace.

    Frequently Asked Questions

    What size furnace for 1,500 sq ft?

    In a cold climate with average insulation: 1,500 × 35 / 0.96 ≈ 55,000 BTU input. A 60,000 BTU furnace is the right standard size.

    What is AFUE and why does it matter?

    AFUE (Annual Fuel Utilization Efficiency) is the ratio of heat output to fuel consumed. A 96% AFUE furnace wastes 4 cents of every dollar of gas. Higher AFUE means lower energy bills — high-efficiency models cost more upfront but pay back in 5–8 years.

    Is an oversized furnace a problem?

    Yes. Oversized furnaces short-cycle — they reach setpoint quickly and shut off, then restart. This causes temperature swings, excess humidity, more wear, and higher bills. Match the furnace to your heat loss calculation.

    Should I get an 80% or 96% AFUE furnace?

    In cold climates where you run the furnace heavily, 96% pays back faster. In mild climates with short heating seasons, the payback may exceed 10 years. Also consider that 80% furnaces vent through B-vent pipe while 96% use PVC — this affects installation cost.

    How is furnace sizing different from heat pump sizing?

    Furnaces are sized in BTU/h input. Heat pumps are sized in tons or BTU/h output. A furnace delivers heat directly from combustion; a heat pump moves heat from outside, so its efficiency varies with outdoor temperature.

    Comparing a heat pump to a furnace? Use the Heat Pump Size Calculator or read our heat pump vs furnace comparison.

    Typical Results

    1,000 sq ft, mild(average insulation, 96% AFUE)40,000–60,000 BTU/h
    1,500 sq ft, cold(average insulation, 96% AFUE)60,000–80,000 BTU/h
    2,000 sq ft, cold(average insulation, 96% AFUE)80,000–100,000 BTU/h
    2,500 sq ft, v.cold(average insulation, 96% AFUE)120,000–150,000 BTU/h

    💡 A Manual J load calculation from a licensed HVAC contractor is the only reliable way to size a furnace. This calculator provides an estimate for planning purposes.

    Common Mistakes

    • •Sizing based on the old furnace nameplate without checking if it was already oversized
    • •Ignoring duct system capacity — an oversized furnace overwhelms undersized ducts
    • •Not accounting for new windows or insulation upgrades that reduce the heat load

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