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
- This is a rule-of-thumb planning estimate. An HVAC contractor's Manual J calculation, the equipment's rating and local code govern the final size.
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.
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 can run one in an hour with software; ask for the report and check that the inputs match your house rather than a default.
Also compare the furnace to the ductwork. A larger furnace needs more airflow, roughly 400 cubic feet per minute per ton of cooling on a shared blower, and undersized ducts will make it noisy and trip the high-limit switch. Two-stage and modulating furnaces forgive a little oversizing because they run at partial output most of the time.
Key facts
- Residential gas furnaces are labeled by input, in 20,000 BTU steps from 40,000 to 140,000 BTU per hour; 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.
- The DOE climate zones run from zone 1 in south Florida to zone 7 in northern Minnesota, and 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.
How many BTU per square foot for heating?
About 30 to 35 BTU per square foot in warm climates, 45 to 50 in the middle of the country and 60 in the coldest zones, before adjusting for insulation and ceiling height.
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
Next steps for this project
The usual order for a insulation & hvac project. See all insulation & hvac calculators.
Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.