ComfortSizer
Gas furnace sizing

Furnace Size Calculator

What size furnace do you need? Set your home size, climate zone, insulation and furnace efficiency. We show the heat you need and the furnace input size that delivers it.

sq ft
ft
Your climate zone

Insulation
Furnace efficiency (AFUE)
Furnace input needed –BTU input
How we got this number

    100% private: the math runs in your browser and nothing is sent anywhere. Estimates for one room or open space up to 2,500 sq ft.

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    Furnace sizing in two steps

    1. Heat your home needs. We use the contractor rule of thumb: square feet × BTU per square foot for your climate (about 30–35 in hot zones up to 50–60 in cold ones), adjusted for insulation and ceiling height.

    2. Furnace input that delivers it. Furnaces are labeled by input, the gas they burn. The heat delivered is the input multiplied by the efficiency (AFUE). So the input you need is the heat output divided by the AFUE: 76,000 BTU of heat from a 96% furnace needs about 79,200 BTU of input, which rounds up to an 80,000 BTU furnace.

    Common furnace sizes

    Residential gas furnaces are commonly sold at 40,000, 60,000, 80,000, 100,000 and 120,000 BTU input. Pick the next size up from the calculator's result, not two sizes up.

    Replacing an old furnace? Do not simply copy its size. Many older furnaces are oversized, and your home may have new windows or insulation. Ask the installer for a Manual J load calculation.

    Heating a single room instead? Use the heating BTU calculator, which also converts the result into space heater watts.

    FAQ

    Questions

    What size furnace do I need for 2,000 square feet?
    With average insulation, about 65,000 BTU of heat in a hot climate, 95,000 in a cool climate and 110,000 in a cold one. Divide by the AFUE for the input size: in a cool climate a 96% furnace needs about 99,000 BTU input, so a 100,000 BTU model.
    What size furnace do I need for 1,500 square feet?
    About 48,750 BTU of heat in a hot climate, 71,250 in a cool climate and 82,500 in a cold one, with average insulation. In a cool climate with a 96% furnace that is about 74,200 BTU input, so an 80,000 BTU furnace.
    What does AFUE mean?
    Annual Fuel Utilization Efficiency: the share of the fuel's energy that becomes heat in your home. A 96% AFUE furnace turns 96 of every 100 BTU of gas into heat.
    Is a bigger furnace better?
    No. An oversized furnace short-cycles, heats unevenly and wears out faster. The next standard size above your load is the right choice.
    Is this a Manual J calculation?
    No. It is a planning estimate. A Manual J load calculation is the accurate way to size a furnace for your specific home.