Furnace Size Calculator – Calculate BTU for Your Home
Estimate heating load and furnace input BTU/hr from heated square footage, US climate zone, insulation, ceiling height, and AFUE. The result is a planning estimate calculated in your browser — use ACCA Manual J before you buy equipment.
What size furnace do I need?
Furnace size depends on more than square footage. The heating load changes with climate, insulation, ceiling height, windows, air leakage, and — when the ducts run through an attic or crawlspace — duct location. Furnace efficiency then converts that load into the input rating on the nameplate.
Use the estimate above as a planning check. ACCA Manual J should be used for final equipment sizing. A heat pump size calculator is the right comparison if you are choosing a heat pump instead of a gas furnace. A single room belongs on the BTU room calculator, not in a whole-home furnace result.
Furnace size by square footage
A 1,000, 1,500, 2,000, 2,500, or 3,000 sq ft house does not have one furnace size. The table uses the same planning model as the calculator: average insulation, 8 ft ceilings, and the other quick-mode defaults. Each cell is heating load, which is the required furnace output — not the input rating.
| Heated area | Zone 2 — hot | Zone 4 — mixed | Zone 6 — cold |
|---|---|---|---|
| 500 sq ft | 6,125 BTU/hr | 9,625 BTU/hr | 14,000 BTU/hr |
| 750 sq ft | 9,188 BTU/hr | 14,438 BTU/hr | 21,000 BTU/hr |
| 1,000 sq ft | 12,250 BTU/hr | 19,250 BTU/hr | 28,000 BTU/hr |
| 1,250 sq ft | 15,313 BTU/hr | 24,063 BTU/hr | 35,000 BTU/hr |
| 1,500 sq ft | 18,375 BTU/hr | 28,875 BTU/hr | 42,000 BTU/hr |
| 2,000 sq ft | 24,500 BTU/hr | 38,500 BTU/hr | 56,000 BTU/hr |
| 2,500 sq ft | 30,625 BTU/hr | 48,125 BTU/hr | 70,000 BTU/hr |
| 3,000 sq ft | 36,750 BTU/hr | 57,750 BTU/hr | 84,000 BTU/hr |
| 4,000 sq ft | 49,000 BTU/hr | 77,000 BTU/hr | 112,000 BTU/hr |
At 95% AFUE, a 2,000 sq ft mixed-climate home in this table needs about 38,500 BTU/hr of output and about 40,526 BTU/hr of furnace input. Divide any load by the AFUE (as a decimal) to get input.
How the furnace BTU calculation works
The quick estimate, with advanced options left at their defaults, is:
Heating BTU ≈ Area × Climate Factor × Insulation Factor × Ceiling Factor
Climate Factor = 0.35 × (indoor °F − outdoor design °F). At the default 70°F indoor temperature, that is the BTU/hr per square foot in the table below. Ceiling Factor = 1 + 0.5 × (ceiling height ÷ 8 − 1), so height raises the load by half of the increase above 8 ft.
The 0.35 coefficient is an EverydayTools planning value for an average envelope. It is not an ACCA Manual J factor and it is not a DOE published number. Outdoor temperatures are planning defaults for each IECC zone, not weather-station data. Replace the outdoor temperature in Advanced options when you know your local design low.
| IECC zone | Planning design low | Climate factor at 70°F indoor |
|---|---|---|
| Zone 1 — very hot | 45°F | 8.75 BTU/hr per sq ft |
| Zone 2 — hot | 35°F | 12.25 BTU/hr per sq ft |
| Zone 3 — warm | 25°F | 15.75 BTU/hr per sq ft |
| Zone 4 — mixed | 15°F | 19.25 BTU/hr per sq ft |
| Zone 5 — cool | 0°F | 24.5 BTU/hr per sq ft |
| Zone 6 — cold | -10°F | 28 BTU/hr per sq ft |
| Zone 7 — very cold | -20°F | 31.5 BTU/hr per sq ft |
| Zone 8 — subarctic | -40°F | 38.5 BTU/hr per sq ft |
How to use the estimate
- Enter heated floor area. Type square feet, or switch to length × width. Count only the floor area the furnace heats.
- Choose climate and insulation. Pick the IECC climate zone closest to your region and how well the home is insulated. Set a local outdoor design temperature under Advanced options if you have one.
- Set ceiling height and AFUE. Ceiling height adjusts the planning load. AFUE converts required heat output into the furnace input rating printed on the nameplate.
- Read load, output, and input. Compare the heating load, the output the furnace must deliver, and the input rating at your AFUE. Ask for an ACCA Manual J calculation before you buy equipment.
Furnace input vs output BTU
Furnace output is the heat delivered to the house. Furnace input is the fuel energy burned each hour. Gas furnace cartons are usually labeled with input. What heats the rooms is output.
Furnace input = required heat output ÷ AFUE
An 80,000 BTU/hr output at 95% AFUE needs about 84,211 BTU/hr of input. An 80% furnace needs a higher input rating than a 95% furnace to deliver the same heat. Common gas input sizes are 40,000, 60,000, 80,000, 100,000, 120,000, and 140,000 BTU/hr. The calculator picks the smallest of those that covers the required input, and it flags the result when that cabinet’s output is more than 140% of the planning load.
What AFUE means for furnace size
AFUE, Annual Fuel Utilization Efficiency, is the seasonal efficiency of a fuel-burning furnace. It does not change the heating load. It changes the input rating required to meet that load. Eighty percent is a common standard-efficiency gas furnace. Ninety-five percent is a common condensing furnace.
Electric resistance heat is different: nearly all of the electricity becomes heat, so input and output are about the same. The 100% choice is for that case. It is not an AFUE rating. Operating cost for a portable heater is a separate question — use the space heater cost calculator for a plug-in unit, not this furnace size.
Worked furnace sizing examples
Each example uses the calculator’s planning model. None of them is a universal BTU-per-square-foot rule.
1,000 sq ft in a warm climate
1,000 sq ft, IECC zone 3, good insulation, 8 ft ceilings, 95% AFUE.
- Heating load
- 11,812 BTU/hr
- Required output
- 11,812 BTU/hr
- Input at 95% AFUE
- 12,434 BTU/hr
- Suggested size
- 40,000 BTU input
The heating load is about 11.8 BTU/hr per sq ft because the climate is mild and the envelope is good. A 40,000 BTU input furnace still delivers far more heat than this planning load.
1,500 sq ft with an 80% gas furnace
1,500 sq ft, IECC zone 4, average insulation, 8 ft ceilings, 80% AFUE.
- Heating load
- 28,875 BTU/hr
- Required output
- 28,875 BTU/hr
- Input at 80% AFUE
- 36,094 BTU/hr
- Suggested size
- 40,000 BTU input
Output stays 28,875 BTU/hr. At 80% AFUE the input rises to 36,094 BTU/hr, because a standard-efficiency furnace must burn more fuel to deliver the same heat.
2,000 sq ft in a cool climate
2,000 sq ft, IECC zone 5, average insulation, 9 ft ceilings, 95% AFUE.
- Heating load
- 52,063 BTU/hr
- Required output
- 52,063 BTU/hr
- Input at 95% AFUE
- 54,803 BTU/hr
- Suggested size
- 60,000 BTU input
The cooler design temperature and 9 ft ceilings lift the load to 52,063 BTU/hr. At 95% AFUE that points to a 60,000 BTU input furnace, which delivers about 57,000 BTU/hr.
2,500 sq ft, cold climate, poor insulation
2,500 sq ft, IECC zone 6, poor insulation, 8 ft ceilings, 80% AFUE.
- Heating load
- 101,500 BTU/hr
- Required output
- 101,500 BTU/hr
- Input at 80% AFUE
- 126,875 BTU/hr
- Suggested size
- 140,000 BTU input
The same square footage in a cold, poorly insulated house is a different furnace. This planning load is 101,500 BTU/hr, and 80% AFUE pushes the input to 126,875 BTU/hr.
What changes furnace size
Climate
A colder design temperature widens the gap between the indoor setpoint and the outdoor design low, so the same house needs more heat. Zone 2, zone 4, and zone 6 in the square-footage table are the same insulation and ceiling — only the climate factor changes. The zone list follows IECC climate zones used across the United States. Design lows in the calculator are planning defaults you can override.
Insulation
For 2,000 sq ft in zone 4 with 8 ft ceilings and 95% AFUE:
| Insulation | Factor | Heating load | Input at 95% |
|---|---|---|---|
| Poor | 1.45 | 55,825 BTU/hr | 58,763 BTU/hr |
| Average | 1 | 38,500 BTU/hr | 40,526 BTU/hr |
| Good | 0.75 | 28,875 BTU/hr | 30,395 BTU/hr |
| Excellent | 0.55 | 21,175 BTU/hr | 22,289 BTU/hr |
Ceiling height
Same 2,000 sq ft zone 4 home, average insulation:
- 8 ft (factor 1): 38,500 BTU/hr
- 9 ft (factor 1.063): 40,906 BTU/hr
- 10 ft (factor 1.125): 43,313 BTU/hr
- 12 ft (factor 1.25): 48,125 BTU/hr
Windows, ducts, and the rest of the envelope
Advanced options apply extra planning multipliers. Two stories use 0.92 and three stories use 0.86, because roof and foundation area is shared across more floor area. A vented crawlspace uses 1.08. Large window area uses 1.14. Ducts in an attic or crawlspace use 1.12. A leaky envelope uses 1.18; a tight one uses 0.86.
After the heating load is known, duct airflow is a separate step. Use the duct CFM calculator and the duct size calculator for airflow. Keep heating BTU out of a cooling-ton estimate. Seasonal electricity for a window unit belongs on the window AC cost calculator.
Common furnace sizing mistakes
Sizing from square footage alone
Two homes with the same floor area can need very different furnaces. Climate, insulation, ceiling height, windows, and ducts all change the heat loss.
Mixing up input and output BTU
The number on many gas furnace labels is input — fuel burned per hour. The house needs output. Output is input times AFUE, so a quoted input rating can look larger than the heating load.
Ignoring insulation
On a 2,000 sq ft mixed-climate home in this model, poor insulation estimates 55,825 BTU/hr and good insulation estimates 28,875 BTU/hr, both at 8 ft ceilings.
Ignoring ceiling height
An 8 ft ceiling is the baseline. Taller rooms add wall area. This model applies half of the height increase, so a 10 ft ceiling is 12.5% above the 8 ft load, not a full 25%.
Assuming a larger furnace is always better
Extra capacity short-cycles, can feel less even, and uses more fuel. A two-stage or modulating furnace is often a better answer than jumping two nameplate sizes.
Treating this estimate as Manual J
This is a planning estimate. It does not measure your windows, leakage, or duct losses, and it is not an ACCA Manual J load calculation.
Comparing heating BTU with air-conditioner tonnage
Furnace BTU/hr is a heating number. Cooling tons are a separate load. A cold-climate house often needs more heating capacity than its cooling system would imply.
When a Manual J calculation is needed
This tool is a planning estimate and a quick furnace-size estimate. It is not a substitute for an ACCA Manual J load calculation. Manual J accounts for each surface, the local design temperature, infiltration, ventilation, and ducts. Building codes and many rebate or warranty programs expect that calculation before equipment is selected.
If you already have a Manual J heat-loss number, enter it under Advanced options. The page will divide that load by your AFUE and show the furnace input. It will not claim to have performed Manual J. Final equipment sizing should be professionally verified.
Furnace sizing questions
What size furnace do I need?
It depends on heated area, climate, insulation, ceiling height, windows, ducts, and air leakage — not square footage alone. This calculator estimates the heating load in BTU/hr, treats that load as the required furnace output, then divides by AFUE to get input. Use the result to compare quotes, and use ACCA Manual J before you buy.
How many BTUs do I need for my house?
There is no single BTU number for a house. In this planning model, average insulation and 8 ft ceilings come out near 12.3 BTU/hr per sq ft in a hot climate (zone 2), 19.3 in a mixed climate (zone 4), and 28 in a cold climate (zone 6). Enter your area and conditions for a home-specific estimate.
How many BTUs do I need for 1,000 sq ft?
For 1,000 sq ft in a mixed climate (IECC zone 4), with average insulation and 8 ft ceilings, this planning model estimates about 19,250 BTU/hr of heating load, or about 19.3 BTU/hr per sq ft. At 95% AFUE the furnace input is about 20,263 BTU/hr. A hotter climate, a colder climate, or a different envelope changes that result. The square-footage table on this page shows the spread.
How many BTUs do I need for 1,500 sq ft?
For 1,500 sq ft in a mixed climate (IECC zone 4), with average insulation and 8 ft ceilings, this planning model estimates about 28,875 BTU/hr of heating load, or about 19.3 BTU/hr per sq ft. At 95% AFUE the furnace input is about 30,395 BTU/hr. A hotter climate, a colder climate, or a different envelope changes that result. The square-footage table on this page shows the spread.
How many BTUs do I need for 2,000 sq ft?
For 2,000 sq ft in a mixed climate (IECC zone 4), with average insulation and 8 ft ceilings, this planning model estimates about 38,500 BTU/hr of heating load, or about 19.3 BTU/hr per sq ft. At 95% AFUE the furnace input is about 40,526 BTU/hr. A hotter climate, a colder climate, or a different envelope changes that result. The square-footage table on this page shows the spread.
How many BTUs do I need for 2,500 sq ft?
For 2,500 sq ft in a mixed climate (IECC zone 4), with average insulation and 8 ft ceilings, this planning model estimates about 48,125 BTU/hr of heating load, or about 19.3 BTU/hr per sq ft. At 95% AFUE the furnace input is about 50,658 BTU/hr. A hotter climate, a colder climate, or a different envelope changes that result. The square-footage table on this page shows the spread.
How many BTU per square foot do I need?
At average insulation and 8 ft ceilings, this planning model uses about 12.3 BTU/hr per sq ft in zone 2, 19.3 in zone 4, and 28 in zone 6, before other adjustments. Poor insulation multiplies those figures by 1.45; good insulation multiplies them by 0.75. They are EverydayTools planning factors, not official ACCA or DOE values.
What is furnace input vs output BTU?
Output is the heat delivered to the house. Input is the fuel energy the furnace burns. Output = input × AFUE, so input = output ÷ AFUE. An 80,000 BTU/hr output at 95% AFUE needs about 84,211 BTU/hr of input. Match output to the heating load, then read the input rating on the nameplate.
What does AFUE mean?
AFUE (Annual Fuel Utilization Efficiency) is the seasonal efficiency of a fuel-burning furnace. An 80% furnace delivers about 80% of its input as heat; a 95% furnace delivers about 95%. AFUE does not change how much heat the house loses. It changes how large the input rating must be to cover that loss. The 100% option is for electric resistance heat, which is not an AFUE rating.
Does insulation affect furnace size?
Yes. For 2,000 sq ft in zone 4 with 8 ft ceilings, this model estimates 55,825 BTU/hr with poor insulation and 28,875 BTU/hr with good insulation. A tighter, better-insulated house needs a smaller furnace, not a larger one.
Does ceiling height affect furnace size?
Yes. The baseline is 8 ft. A 10 ft ceiling on a 2,000 sq ft zone 4 home with average insulation raises the planning load from 38,500 BTU/hr to 43,313 BTU/hr. The model adds half of the height increase, because taller walls add exposed area while the floor and roof do not grow with the ceiling.
Is a furnace BTU calculator accurate?
It is a planning estimate. It uses a simplified heat-loss coefficient, climate-zone design defaults, and adjustment factors you choose. It does not measure your glass, infiltration, or duct leakage, and it is not an ACCA Manual J calculation. Use it to sanity-check a quote, then confirm the load before you purchase equipment.
Do I need a Manual J calculation?
Yes, before you buy or replace a furnace, pull a permit, or size equipment for a house with unusual glass, an addition, or more than one zone. You can enter an existing Manual J heat-loss figure in Advanced options to convert it to furnace input at your AFUE. This page will not create that Manual J for you.
Related heating and cooling tools
Whole-home heating stays on this page. For one room’s cooling or heating BTU, use the BTU room calculator. For a heat pump instead of a furnace, use the heat pump size calculator. Duct planning continues with the duct CFM calculator and the duct size calculator.
Running cost is separate from size. Estimate a window AC’s electricity cost or a space heater’s operating cost on those tools. More home calculators are in Calculator Tools.