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Furnace Size Calculator
Use this free furnace size calculator to find the right heating capacity for a home. Enter the floor area, ceiling height, climate zone, insulation level, and how many windows the home has, and it gives a recommended BTU range plus the same size in kilowatts.
How it works
A furnace is sized by how much heat a home loses on the coldest day of the year, and the starting point is the floor area times a heating load per square foot. That load depends on climate: a home in a hot southern zone needs only about 30 BTU per square foot, while a home in a very cold northern zone needs about 60. The calculator uses 30, 35, 40, 50, and 60 BTU per square foot for zones 1 through 5, which covers the usual range from Florida to Minnesota.
Three adjustments then refine the base number. Insulation matters most: poor insulation raises the load by 15 percent, average insulation leaves it unchanged, and good insulation — sealed attic, insulated walls, and tight windows — cuts it by 10 percent. Windows lose heat faster than walls, so a home with many large windows adds 10 percent, while a home with few windows subtracts 5 percent. Ceiling height matters too, because taller rooms hold more air to heat: each foot above the standard 8 feet adds 5 percent, and each foot below subtracts 5 percent.
The result is a range, not a single number, because no two homes lose heat exactly alike. The calculator shows the midpoint as the headline size and a range 10 percent either side of it, so you can compare furnace models and pick one that lands inside the range. It also converts the result to kilowatts, since some high-efficiency and electric furnaces are rated in kW — divide BTU per hour by 3,412 to convert.
Avoid oversizing. A furnace far larger than the home needs will heat the house in short bursts, switching on and off constantly, which wears parts out, leaves cold spots, and wastes fuel. A furnace slightly below the top of the range is usually the better pick. This calculator is a planning estimate; a heating contractor's Manual J load calculation is the formal sizing method used before a real installation.
Worked example
A 2,000 sq ft home in Chicago has 9-foot ceilings, average insulation, and an average number of windows. Chicago sits in Zone 4 (Cold), so the base load is 50 BTU per square foot.
Step 1: Base load — 2,000 × 50 = 100,000 BTU/hr.
Step 2: Insulation and windows — both average, so no change: 100,000 × 1.00 × 1.00 = 100,000 BTU/hr.
Step 3: Ceiling height — 1 + (9 − 8) × 0.05 = 1.05, so 100,000 × 1.05 = 105,000 BTU/hr.
Step 4: Range — 105,000 × 0.90 = 94,500 and 105,000 × 1.10 = 115,500 BTU/hr. In kilowatts: 105,000 ÷ 3,412 = 30.8 kW.
The home needs a furnace of about 105,000 BTU/hr, and any model rated between 94,500 and 115,500 BTU/hr (27.7 to 33.9 kW) is a good fit — a standard 100,000 BTU furnace sits right in the middle of that range.
FAQ
What climate zone is my home in?
Zone 1 (Hot) covers the far south such as southern Florida and Texas; Zone 2 (Warm) covers the Gulf Coast and southern Arizona; Zone 3 (Moderate) covers much of the Southeast and mid-Atlantic; Zone 4 (Cold) covers the Midwest and Northeast; Zone 5 (Very Cold) covers the northern tier, including Minnesota, North Dakota, and Maine.
Is a bigger furnace better?
No. An oversized furnace heats the home in short bursts, cycles on and off constantly, wears out parts faster, and leaves uneven temperatures. Pick a furnace inside the calculator's recommended range, near the middle, rather than the largest model available.
How do I convert BTU to kW?
Divide BTU per hour by 3,412. For example, a 105,000 BTU/hr furnace is about 30.8 kW. The calculator shows both figures so you can compare gas furnaces rated in BTU with electric models rated in kW.
Does ceiling height really change furnace size?
Yes. Taller ceilings mean more air volume to heat. Each foot above the standard 8 feet adds about 5 percent to the load in this calculator, so a home with 10-foot ceilings needs about 10 percent more heating capacity than the same floor plan with 8-foot ceilings.
Should I replace my furnace with the size this calculator gives?
Use it as a planning estimate and a check on contractor quotes. Before buying, ask a heating contractor for a Manual J load calculation, which measures the actual home and is the standard sizing method for a real installation.