Furnace Size Calculator
Calculate what size furnace a house needs in BTU per hour from its square footage, climate zone, insulation and ceiling height, matched to standard 80% or 95% AFUE sizes.
By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.
Results
- Heating output needed
- 90,000 BTU per hour
- For 2,000 sq ft with 8 ft ceilings
- Furnace input rating to buy
- 112,500 BTU per hour
- Output / 80% AFUE; furnaces are labeled by input
- Closest standard size
- 120,000 BTU input
- Delivers 96,000 BTU per hour of output at 80% AFUE
- BTU per sq ft used
- 45 BTU per sq ft
- 45 for the climate zone, adjusted 0% for insulation and 0% for ceiling height
- Before you buy
- Manual J load calculation
- Rule-of-thumb planning estimate. IRC M1401.3 requires Manual J and S sizing for the permit; ask the contractor for the report and check the inputs against your house.
Furnace Size formula
BTU per sq ft = Climate factor (30 to 60) x insulation factor (1.15, 1, 0.85) x Ceiling height / 8
Heating output = Floor area (sq ft) x BTU per sq ft
Furnace input = Heating output / AFUE (0.80 or 0.95)
Round up to the next standard input: 40,000, 60,000, 80,000, 100,000, 120,000 or 140,000 BTUWorked example
A 1,500 sq ft house in cold zone 6 with good insulation, 9 ft ceilings and a 95% AFUE furnace: 60 BTU per sq ft x 0.85 for good insulation x 9 / 8 for the ceilings = 57.4 BTU per sq ft. 1,500 x 57.4 = 86,063 BTU per hour of output.
Divide by 0.95 for the input rating: 86,063 / 0.95 = 90,592 BTU. The next standard size is a 100,000 BTU input furnace, which delivers 95,000 BTU per hour at 95% AFUE, about 10% above the estimated load.
Quick reference
| Climate | Output needed | Input at 80% AFUE | Furnace at 80% | Input at 95% AFUE | Furnace at 95% |
|---|---|---|---|---|---|
| Zone 1 to 2, hot | 60,000 BTU | 75,000 BTU | 80,000 BTU | 63,158 BTU | 80,000 BTU |
| Zone 3, warm | 70,000 BTU | 87,500 BTU | 100,000 BTU | 73,684 BTU | 80,000 BTU |
| Zone 4, mixed | 90,000 BTU | 112,500 BTU | 120,000 BTU | 94,737 BTU | 100,000 BTU |
| Zone 5, cool | 100,000 BTU | 125,000 BTU | 140,000 BTU | 105,263 BTU | 120,000 BTU |
| Zone 6 to 7, cold | 120,000 BTU | 150,000 BTU | Over 140,000 BTU | 126,316 BTU | 140,000 BTU |
How to calculate furnace size
Start with the floor area the furnace will actually heat. Multiply by a BTU per square foot factor for your climate: about 30 in the hot South, 45 in the mid-Atlantic and Ohio Valley, and 60 in the northern tier. Bump the result up 15 percent for an older, drafty house with single-pane windows, or down 15 percent for a tight, well insulated one, and scale for ceilings taller than 8 ft since the furnace heats volume, not floor.
The number you get is the heat the house loses on a design cold day, so it is the output the furnace must deliver. Furnaces are sold by input, so divide by the AFUE: an 80% furnace burns 100,000 BTU of gas to deliver 80,000 BTU of heat. Then round up to the next standard size, but only one step; ACCA Manual S allows a furnace output of 100 to 140 percent of the heating load. Some brands step 45, 60, 75, 90, 105 and 120 (Rheem, Ruud) or 44, 66, 88 and 110 (Lennox), so compare output, not the model number.
Why a Manual J still matters
Square-footage rules cannot see how many windows face west, whether the ducts run through a hot attic, or how leaky the house is. ACCA Manual J accounts for all of that, room by room, and it is what the code and most equipment warranties expect. A contractor runs one in about an hour with software, typically $150 to $400 on its own and free with most bids. Ask for the report and check that the window count, insulation and design temperature match your house rather than a default.
Also compare the furnace to the ductwork. Airflow is output divided by 1.08 times the temperature rise on the nameplate: 80,000 BTU of output at a 50 F rise needs about 1,480 CFM, which a single 16 in return cannot move. Starved ducts run the blower at high static, trip the high limit and crack the heat exchanger years early; a 100,000 BTU furnace usually wants two returns or a 20 x 25 in filter grille. Two-stage and modulating furnaces forgive a little oversizing because they run at partial output most of the time.
What this calculator leaves out
Not included: the Manual J and S, the mechanical and gas permits (typically $50 to $250) and inspection, and the venting: B-vent or a chimney liner for 80%, two PVC pipes through the wall for 95%, plus a liner for a water heater left alone on the chimney. Nor the condensate drain, pump or neutralizer, a larger gas line if input rises, the thermostat, and duct or return changes. The estimate also ignores solar gain, duct losses in an attic or crawlspace, and the rooms far from the thermostat that a Manual J sizes room by room.
Mistakes that cost money
- Ordering by output: the label number is input; a 100,000 BTU 80% furnace delivers 80,000, so a 90,000 BTU load needs the 120,000 model.
- Matching the old furnace: 1980s units were sized 50 to 100% over; a Manual J on the house today usually drops a size.
- Oversized furnace on old ducts: 1,500 CFM will not move through a 16 in return, the high limit trips and the heat exchanger cracks early.
- Going 95% and leaving the water heater alone on the masonry chimney: the orphaned flue condenses and spalls; add a liner (IRC G2427).
- Skipping the mechanical permit: inspectors check Manual J and S (IRC M1401.3), venting and combustion air, and unpermitted swaps surface at resale.
- A 95% furnace makes several gallons of acidic condensate a day; without a drain or pump and, where required, a neutralizer, it floods the closet.
Key facts
- Residential gas furnaces are labeled by input; most lines step 40,000, 60,000, 80,000, 100,000, 120,000 and 140,000 BTU per hour, some 45, 60, 75, 90, 105 and 120 thousand, and output is input times AFUE.
- The federal minimum for a new gas furnace is 80% AFUE; condensing furnaces reach 90 to 98% and need a PVC vent and a condensate drain.
- IECC climate zones run from zone 1 in south Florida to zone 8 in interior Alaska. The rule-of-thumb heating load roughly doubles from the hottest zone to the coldest.
- ACCA Manual J is the residential load calculation standard that the IRC requires for sizing heating and cooling equipment.
- An oversized furnace short-cycles, which wastes gas, wears the igniter and blower and leaves rooms far from the thermostat cold.
Frequently asked questions
What size furnace do I need for a 2,000 sq ft house?
In a mixed zone 4 climate with average insulation, about 90,000 BTU per hour of output, which is a 120,000 BTU input furnace at 80% AFUE or a 100,000 BTU furnace at 95% AFUE. In a hot climate a 2,000 sq ft house needs only about 60,000 BTU of output.
Do I need a permit to replace my furnace?
Nearly everywhere, yes: a mechanical permit and often a gas permit, typically $50 to $250 together. The inspector checks the Manual J and S sizing (IRC M1401.3), the vent and clearances, combustion air and the gas shutoff, plus the condensate drain on a 95% unit. A licensed contractor pulls it as part of the bid. A swap done without one shows up at resale when the buyer's inspector finds no record.
Is furnace size input or output?
The number on the label is input, the gas burned. Multiply by the AFUE for output: an 80,000 BTU furnace at 80% AFUE delivers 64,000 BTU per hour, and at 95% AFUE it delivers 76,000.
Is it better to oversize a furnace?
No. Round up only to the next standard size. A furnace that is far too big cycles on and off, wears out sooner, wastes fuel and heats unevenly. If the estimate lands just above a size, a Manual J will usually show the smaller one is enough.
Sources and references
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Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.