How the calculator sizes a furnace
A furnace has to replace the heat your house loses on a very cold night, the "design day." That loss is measured in BTU per hour. The calculator estimates it from floor area and climate zone, adjusts for how the house is built, then finds the smallest stock furnace whose output covers it. Here is the math for the default house.
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1
Start from the climate zone
Zone 4 (New York, Washington DC, Seattle) has a design temperature near 15°F, or 55 degrees below a 70°F room. A typical 1980–2005 house there loses about 22 BTU/h per square foot.
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2
Multiply by the floor area
2,000 sq ft × 22 = 44,000 BTU/h.
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3
Adjust for age, insulation, windows and ceilings
Typical insulation, average windows and 8 ft ceilings leave it at 44,000 BTU/h, or 12.9 kW. A pre-1980 house would add 30%. Each foot of ceiling above 8 ft adds 6%.
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4
Convert to furnace input
Furnaces are sold by input, the fuel they burn. At 96% AFUE you need 44,000 ÷ 0.96 = 45,833 BTU/h of input. The next stock size is 60,000, which puts out about 57,600 BTU/h, 31% over the loss.
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5
Estimate the yearly fuel
44,000 ÷ 55°F = 800 BTU/h per degree. × 4,500 heating degree days × 24 hours × 0.6 = 51.8 million BTU of heat a year. ÷ 0.96 = 540 therms. At $1.48 a therm that is about $800 a year.
Heat loss per square foot by climate zone
The base rate is about 0.4 BTU/h per square foot for each degree between 70°F inside and the zone's design temperature. That fits a 1980–2005 house with typical insulation. Tap a zone to load it.
| Zone | Design | BTU/h per sq ft | 2,000 sq ft | HDD |
|---|---|---|---|---|
| 1 Miami | 47°F | 9 | 18,000 | 150 |
| 2 Houston | 32°F | 15 | 30,000 | 1,400 |
| 3 Atlanta | 24°F | 18 | 36,000 | 2,700 |
| 4 New York | 15°F | 22 | 44,000 | 4,500 |
| 5 Chicago | 0°F | 28 | 56,000 | 6,300 |
| 6 Minneapolis | −11°F | 32 | 64,000 | 7,800 |
| 7 Duluth | −17°F | 35 | 70,000 | 9,800 |
Design temperatures are rounded values for the example city: the temperature it stays above 99% of the hours in a year. Heating degree days (HDD, base 65°F) are typical for the zone and drive the yearly fuel only, not the size. If your utility bill or a weather site lists your own heating degree days, type them in.
You will see older charts online that say 45 to 60 BTU per square foot for cold climates. Those numbers describe drafty houses and pile safety margins on top of each other. A modern Manual J usually lands well below them, which is why sizing from such a chart tends to buy too much furnace.
What each adjustment does
| Factor | Change | Why |
|---|---|---|
| Built before 1980 | +30% | Little wall insulation, leaky windows and frames |
| Built 2006 or later | −20% | Built to modern energy codes |
| Poor / upgraded insulation | +15% / −10% | Attic, walls and air sealing compared with the house's age |
| Small / large window area | −5% / +10% | Glass loses heat several times faster than a wall |
| Each foot of ceiling over 8 ft | +6% | More wall area and more air leaking out |
These factors are HomeOutlay estimates, picked to match what load calculations show for typical houses. They can't see a cold basement, a bonus room over the garage or a wall of west glass. That is the job of a room-by-room Manual J.
Input vs output: what AFUE means
AFUE (annual fuel utilization efficiency) is the share of the fuel that ends up as heat in the house over a season. The rest goes up the vent. The federal minimum for most indoor gas furnaces is 80%. Condensing furnaces reach 90% to 98% by pulling extra heat out of the exhaust. They vent through PVC pipe and need a drain for the water they make.
| Input BTU/h | Output at 80% | Output at 96% |
|---|---|---|
| 40,000 | 32,000 | 38,400 |
| 60,000 | 48,000 | 57,600 |
| 80,000 | 64,000 | 76,800 |
| 100,000 | 80,000 | 96,000 |
| 120,000 | 96,000 | 115,200 |
Some product lines also come in 45,000, 70,000, 90,000 or 110,000 BTU input. The furnace's data plate lists its exact output, so check it against the load. Two-stage and modulating furnaces run most of the winter at a lower fire, which keeps rooms more even when the size runs a little big.
A 2023 DOE rule set a 95% AFUE minimum for most new gas furnaces starting in late 2028. In 2026 that rule is still being fought in court, so its future is uncertain. Either way, it doesn't force you to replace a furnace that works.
Why a Manual J beats any rule of thumb
The International Residential Code (section M1401.3) requires heating and cooling equipment to be sized with ACCA Manual S, from loads worked out with ACCA Manual J. Manual J counts each wall, window, door and ceiling, the air leakage and the duct losses, using your town's design temperature. Manual S then picks a furnace whose output covers the load without going far over it. For furnaces, the range most often quoted from Manual S is 100% to 140% of the heating load. The calculator flags anything outside that band.
Bigger is not safer. An oversized furnace short-cycles: it blasts heat, shuts off, then fires again. That wastes fuel, wears parts and makes rooms swing between warm and cool. It also pushes more air than small ducts can carry, so it gets noisy. Matching the old furnace's size is the most common mistake, because many older furnaces were oversized to begin with.
What a furnace costs to buy and run
Trane's 2026 price guide puts an installed 80% gas furnace at about $5,100 to $6,400 and a 96% furnace at about $6,600 to $8,400, depending on single-stage, two-stage or variable-speed models. Those figures cover the unit, labor and basic accessories, but not a new thermostat. New venting, a gas line, ductwork or a tight closet can push a quote higher. The calculator starts at $5,500 for an 80% furnace and $7,000 for 90% and up. Type your bids in.
Fuel is the bigger number over the furnace's 15 to 20 year life. Residential natural gas averaged $15.34 per thousand cubic feet across the U.S. in 2025, or about $1.48 a therm. Prices run from under $0.80 a therm in Idaho to over $2.40 in Massachusetts and Florida, and far more in Hawaii. Pick your state to load its average, or type the rate from your bill (supply and delivery together).
Whether 96% beats 80% comes down to climate and gas price. In the default house the gap is about $160 a year, so a $1,500 premium pays back in about 9 to 10 years. In zone 5 or 6, or with propane, it pays back much sooner. In a mild climate with cheap gas, an 80% furnace can make sense, especially where running a new PVC vent is hard.
Tips before you buy
- Check your gas bills. A year of therms tells you how much heat the house really uses. If your last winter's use is far below the calculator's estimate, your house is tighter than average and a smaller furnace may do.
- Seal and insulate first. Attic insulation and air sealing can cut the load enough to drop a size, which saves money on the furnace and every winter after.
- Size the blower for the AC too. If the furnace also moves air for central AC, the blower must match the cooling tonnage. Your contractor should check both.
- Think about a heat pump. If the AC is old too, a heat pump can replace it and share the heating with the furnace. Compare the running costs with our heat pump savings calculator.
- Hire a licensed pro. Gas piping, venting and combustion air are code items with real carbon monoxide risk. Most areas require a permit and inspection. Put a CO alarm on every level.
Sources and assumptions
- Load calculation and equipment selection: ACCA Manual J and Manual S, as required by IRC section M1401.3.
- Climate zones by county: PNNL, Guide to Determining Climate Zone by County (IECC 2021). The state list picks the zone of each state's largest metro area. Design temperatures are rounded ASHRAE 99% values for the example cities. Degree days per zone are HomeOutlay estimates that fit the IECC zone limits (for example, zone 5 is 5,400 to 7,200 heating degree days).
- Natural gas prices: EIA, Natural Gas Prices, residential price by state, annual 2025 (released September 30, 2026). For Connecticut, Maine, Maryland, North Carolina, Rhode Island and South Carolina, where 2025 is not yet published, the calculator uses 2024. Converted at 1,036 BTU per cubic foot (EIA), or 10.36 therms per thousand cubic feet.
- Propane prices: EIA, Heating Oil and Propane Update, residential propane, week of October 26, 2026 (U.S. $2.73 a gallon, before taxes). States not surveyed use their regional average or the U.S. average. Propane holds 91,452 BTU per gallon.
- Furnace prices: Trane 2026 HVAC Replacement Cost Guide, national installed ranges, checked October 2026. Furnace efficiency basics: DOE Energy Saver, Furnaces and Boilers.
- HomeOutlay assumptions: 70°F indoors; 0.4 BTU/h per square foot per degree for a typical 1980–2005 house; the age, insulation, window and ceiling factors in the table above; yearly heat = heat loss per degree × degree days × 24 × 0.6 (the 0.6 accounts for heat from sun, people and appliances, and for nights that are milder than design); stock input sizes of 40,000 to 120,000 BTU/h. Furnace blower electricity is not included.
Common questions
What size furnace do I need for a 2,000 square foot house?
With average insulation, a 2,000 square foot house loses about 44,000 BTU per hour on a design day in a mixed climate like New York (zone 4) and about 56,000 in a cold one like Chicago (zone 5). That points to a 60,000 BTU input furnace in zone 4. In zone 5 it is 60,000 BTU at 96% AFUE or 80,000 BTU at 80% AFUE. A Manual J load calculation should set the final size.
How many BTUs do I need to heat 1,000 square feet?
About 22,000 BTU per hour in a mixed climate and 28,000 in a cold one, for a house with average insulation and 8 foot ceilings. A drafty pre-1980 house needs about 30% more. The smallest common furnace, 40,000 BTU input, covers most of that range.
What is the difference between furnace input and output BTU?
Input is the fuel the furnace burns per hour. Output is the heat it delivers to the house. Output is roughly input times efficiency, so a 60,000 BTU input furnace delivers about 48,000 BTU at 80% AFUE and about 57,600 BTU at 96% AFUE. Size the furnace on output.
Is a 96% furnace worth it over an 80% furnace?
It depends on climate and gas price. In this calculator's default house, a 2,000 square foot home in zone 4 with gas at $1.48 a therm, a 96% furnace saves about $160 a year. At a price gap of about $1,500, that pays back in about 9 to 10 years. Colder climates and pricier gas pay back faster.
What happens if a furnace is too big?
An oversized furnace heats the house in short bursts and shuts off, then starts again. That short cycling wastes fuel, wears the igniter and heat exchanger sooner, makes rooms swing between warm and cool, and can make the blower loud on small ducts. Pick the smallest size whose output covers the heat loss.
How much gas does a furnace use per year?
For the default 2,000 square foot house in zone 4 with a 96% furnace, about 540 therms a year, or about $800 at $1.48 a therm. The same house near Chicago (zone 5) uses about 760 therms. Real use depends on your thermostat setting, how tight the house is and the winter you get.