Furnace Size Calculator

🔥 What Size Furnace Do I Need? — BTU & AFUE Calculator

Find the right furnace size for your home in BTU. Enter your square footage, climate zone, insulation and fuel type — we calculate the heat output you need and the furnace input rating (adjusted for AFUE efficiency) so you buy the correct 40k, 60k, 80k or 100k BTU unit. For homes in the US, Canada, UK & Europe.

🌍 Prefer metric? Switch units here

📐1. Floor Area & Ceiling

sq ft

Range: 100 – 10,000 sq ft (whole-home or a single addition)

ft

Range: 7 – 20 ft. Taller ceilings need proportionally more heat.

🌍2. Climate Zone

Zones follow US DOE / IECC heating climate zones. Colder zone = more BTU per sq ft. Pick your country above to auto-set it.

🧱3. Insulation

🪟4. Windows / Glazing

💨5. Air Tightness / Drafts

Older, draughty homes lose heat through air leaks and need a bigger furnace. A modern, sealed home needs less.

🔥6. Fuel Type & Furnace Efficiency (AFUE)

% AFUE

Standard gas = 80% · High-efficiency condensing = 90–98% · Electric = 100%. AFUE is the % of fuel turned into usable heat.

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Furnace Size Chart by Square Footage & Climate Zone

Use this quick-reference table to find a starting furnace size (in BTU input, at 80% AFUE) for your home. Then drop your exact details into the calculator above to factor in insulation, ceiling height, windows, air-tightness and your furnace's real AFUE rating.

Home AreaWarm South (Zone 1–2)Moderate (Zone 3–4)Cold North (Zone 5–7)
1,000 sq ft35,000–45,000 BTU45,000–55,000 BTU60,000–75,000 BTU
1,500 sq ft45,000–60,000 BTU60,000–80,000 BTU90,000–110,000 BTU
2,000 sq ft60,000–75,000 BTU80,000–100,000 BTU110,000–140,000 BTU
2,500 sq ft75,000–90,000 BTU100,000–120,000 BTU140,000–160,000 BTU
3,000 sq ft90,000–110,000 BTU120,000–140,000 BTU160,000–180,000 BTU

These are furnace input (nameplate) BTU ranges for a standard 80% AFUE gas furnace and 8 ft ceilings. A high-efficiency 96% AFUE furnace of the same output carries a lower input number. Poor insulation, draughty construction and tall ceilings push you higher — verify with the calculator above.

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How to Size a Furnace: BTU Output vs BTU Input

Furnace sizing has one twist air-conditioner sizing does not: the difference between output and input. Getting this right is what stops you buying the wrong unit.

1. Work out the heat your home loses (BTU output needed)

Start with your heated square footage and multiply by a heating factor for your climate zone — roughly 28 BTU/sq ft in the warm South up to 58 BTU/sq ft in the far North. Then adjust for ceiling height, insulation, windows and how draughty the house is. That number is the heat output (delivered BTU/hr) your furnace must replace on a design-cold day.

2. Divide by AFUE to get the furnace input size

AFUE (Annual Fuel Utilization Efficiency) is the percentage of fuel a furnace turns into usable heat. A standard gas furnace is 80% AFUE — it wastes 20% up the flue. So to deliver 72,000 BTU/hr of heat, an 80% furnace must be rated 72,000 ÷ 0.80 = 90,000 BTU input. A 96% condensing furnace delivering the same heat needs only 75,000 BTU input. The input number is what's on the furnace nameplate and in the store listing.

3. Round up to a standard furnace size

Furnaces are sold in fixed sizes — typically 40k, 60k, 80k, 100k and 120k BTU input. Always round up to the next available size so the furnace can keep up on the coldest nights, but don't grossly oversize: an oversized furnace short-cycles, wears out faster and heats unevenly.

4. Match the fuel type

Gas & propane furnaces run 80–98% AFUE. Oil furnaces are typically 80–87%. Electric furnaces are 100% AFUE (all the electricity becomes heat), so their input equals their output — but electricity usually costs more per BTU, which is why many homeowners compare against a heat pump.

Frequently Asked Questions

What size furnace do I need for a 1,500 sq ft house?
A 1,500 sq ft home typically needs about 60,000–80,000 BTU input in a moderate climate, rising to 90,000–110,000 BTU in cold northern zones, for a standard 80% AFUE gas furnace. A high-efficiency 96% AFUE furnace of the same heat output carries a lower input rating. Use the calculator above to factor in your insulation, ceilings and fuel.
What size furnace do I need for a 2,000 sq ft house?
A 2,000 sq ft home usually needs roughly 80,000–100,000 BTU input (80% AFUE) in a moderate climate, and 110,000–140,000 BTU in a cold climate. Good insulation and a tight, modern building envelope can pull this toward the lower end; draughty older homes push it higher.
What is AFUE and why does it change the furnace size?
AFUE (Annual Fuel Utilization Efficiency) is the share of fuel a furnace converts to usable heat. It changes the input rating you shop for: to deliver the same heat, an 80% AFUE furnace needs a bigger input number than a 96% one. For example, 72,000 BTU of delivered heat needs 90,000 BTU input at 80% AFUE but only 75,000 BTU input at 96% AFUE.
What's the difference between BTU input and BTU output on a furnace?
Input is how much fuel energy the furnace consumes (the nameplate rating you buy by). Output is how much heat it actually delivers into your home. Output = input × AFUE. A 100,000 BTU input furnace at 80% AFUE delivers 80,000 BTU output. Size your home to the output you need, then work back to the input.
Is it bad to oversize a furnace?
Yes. An oversized furnace heats the house too quickly, then shuts off — this short-cycling wastes fuel, wears out the ignitor and blower faster, and leaves uneven, drafty temperatures. Slightly oversizing (rounding up to the next standard size) is fine and normal; grossly oversizing by 50%+ is a common, costly mistake.
How many BTU do I need per square foot for heating?
As a rule of thumb, homes need about 30–35 BTU/sq ft in warm southern climates, 40–45 BTU/sq ft in moderate zones, and 50–60 BTU/sq ft in cold northern climates — measured as delivered heat output. Divide by your furnace's AFUE to get the input rating. The calculator above refines this for your insulation, windows and ceiling height.
Is a bigger furnace always better?
No. The right-sized furnace runs in longer, steady cycles that keep temperatures even and humidity comfortable, and it lasts longer. A too-big furnace blasts heat and shuts off repeatedly. A too-small one runs constantly and can't keep up on the coldest days. Aim for the calculated size rounded up to the nearest standard model.
Should I get a gas furnace or a heat pump?
It depends on your climate and energy prices. Gas furnaces deliver strong, cheap heat in very cold climates with low gas prices. Heat pumps are far more efficient (300%+ effective) and can also cool in summer, making them attractive where electricity is reasonably priced or winters are mild. Compare the running costs side by side with our Heat Pump Savings Calculator.

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Last updated: 2026. Maintained by the Refcon HVAC engineering team. Found an error or have a suggestion? Tell us.

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Explore more furnace sizing pages:
Furnace size by home area: 1,000 sq ft  |  1,500 sq ft  |  2,000 sq ft  |  2,500 sq ft
By city / climate: Chicago  |  Minneapolis  |  Denver  |  Boston
Weighing a heat pump instead? Compare with the Heat Pump Savings Calculator.
Before you decide — three quick things
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Check rebates & credits
High-efficiency systems may qualify for utility rebates and federal or state incentives. Find current programs at ENERGY STAR & DSIRE.
Reviewed for accuracy
Figures are reviewed by the Refcon HVAC engineering team using U.S. DOE, ENERGY STAR & ACCA Manual J data. Estimates only — confirm with a licensed pro.
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