Furnace Size Calculator
A furnace size calculator multiplies sq ft × BTU per sq ft, then divides by AFUE; 1,500 sq ft × 40 = 60,000 BTU/hr, or 63,158 BTU input at 95% efficiency.
Furnace size calculator: square feet × BTU factor ÷ AFUE
This furnace size calculator gives a rule-of-thumb heating size from three inputs. The formula is output (BTU/hr) = heated sq ft × BTU per sq ft, then input (BTU) = output ÷ (AFUE ÷ 100). A 1,500 sq ft house at 40 BTU per sq ft needs 60,000 BTU/hr delivered; a 95% furnace must burn 63,158 BTU of fuel to deliver it.
What each step means
Output. The house leaks heat through walls, windows, the roof, and air leaks. On the coldest days, the furnace must put that heat back. The climate factor is a shortcut for how fast a typical square foot leaks heat in your area.
Input. No furnace turns all its fuel into room heat. The AFUE rating says how much does. Enter AFUE as a percent, such as 95, or as a decimal, such as 0.95; the calculator accepts either.
Furnaces are usually labeled by input BTU. Many residential models also list output, or “heating capacity.” Compare the output figure with your load.
Worked examples
- Default: 1,500 sq ft, 40 BTU/sq ft, 95% AFUE
- Output: 1,500 × 40 = 60,000 BTU/hr
- Input: 60,000 ÷ 0.95 = 63,158 BTU
- Older cold-climate house: 2,200 sq ft, 45 BTU/sq ft, 80% AFUE
- Output: 2,200 × 45 = 99,000 BTU/hr
- Input: 99,000 ÷ 0.80 = 123,750 BTU
The 80% furnace in example 2 burns 24,750 BTU more fuel than it delivers. A 95% unit would need about 104,211 BTU of input for the same output.
Furnace sizing reference table
| Home size | 30 BTU/sq ft (mild) | 40 BTU/sq ft (moderate) | 45 BTU/sq ft (cold) |
|---|---|---|---|
| 1,000 sq ft | 30,000 | 40,000 | 45,000 |
| 1,500 sq ft | 45,000 | 60,000 | 67,500 |
| 2,000 sq ft | 60,000 | 80,000 | 90,000 |
| 3,000 sq ft | 90,000 | 120,000 | 135,000 |
Values are output in BTU/hr. Divide by your AFUE for the input size.
| AFUE | Input needed for 60,000 BTU/hr output |
|---|---|
| 80% | 75,000 BTU |
| 90% | 66,667 BTU |
| 95% | 63,158 BTU |
| 98% | 61,224 BTU |
How to gather the inputs
- Heated area: add the floor area of every heated level. Leave out unheated garages and crawlspaces.
- Climate factor: start at 40. Go lower for a mild winter or a tight, recently insulated home. Go higher for long, cold winters, single-pane windows, or ceilings above 8 ft.
- AFUE: read it from the yellow EnergyGuide label or the furnace data plate.
- Better data: the heat loss calculator estimates loss from walls and windows instead of floor area.
Picking a model from the result
Gas furnaces are commonly sold in steps of roughly 20,000 BTU input, such as 40,000, 60,000, 80,000, and 100,000. Take the output your house needs, then look for the smallest model whose rated output meets it. For the default house, that is a model delivering at least 60,000 BTU/hr. A 95% furnace with 60,000 BTU input delivers only 57,000, so the next step up would be needed.
Two-stage and modulating furnaces run at lower output most of the time and step up only on cold days. They soften the penalty for slight oversizing and keep temperatures steadier.
Common furnace sizing mistakes
- Copying the old furnace size. Older furnaces were often oversized, and the house may have been insulated since.
- Comparing input to load. The load must be met by output, not input.
- Ignoring ductwork. A bigger furnace on small ducts can be noisy and run hot. Airflow matters as much as BTUs.
- Oversizing “to be safe.” Short cycling wastes fuel and wears parts.
- Forgetting the altitude. At high elevations, gas appliances can deliver less heat than their sea-level rating. Ask the installer whether a derate applies where you live.
- Skipping a load calculation. Manual J accounts for windows, insulation, orientation, and air leakage.
Related calculators
Weighing fuels? Try the heat pump vs gas furnace calculator. For a single room or addition, the mini split sizing calculator is a better fit, and the furnace and AC replacement cost calculator helps with the budget.
Sizing here is a planning estimate. Have a licensed HVAC contractor confirm equipment size with a Manual J load calculation.
Frequently asked questions
How do I size a furnace for my home?
Multiply heated square feet by a climate factor in BTU per square foot. This calculator suggests about 30 for mild winters, 40 as a moderate default, and 45 for cold ones. That gives the heat output needed per hour. Divide by the AFUE to get the input size most furnaces are labeled with.
What is the difference between BTU output and fuel input?
Output is the heat delivered to your rooms. Input is the fuel energy burned to make it. A 95% AFUE furnace turns about 95 of every 100 BTU of fuel into delivered heat, so a 60,000 BTU/hr output needs about 63,158 BTU of input. At 80% AFUE the same output needs 75,000.
What climate factor should I use?
It is a rule of thumb for how much heat each square foot loses on a cold day. Use the low end for mild climates or tight, well-insulated houses, and the high end for cold climates, drafty houses, or tall ceilings. A Manual J load calculation replaces this guess with real numbers.
Does a higher AFUE furnace need less fuel?
Yes. A higher AFUE wastes less heat up the flue, so it burns less fuel for the same delivered heat. Whether the higher price pays back depends on your fuel cost, climate, and how long you keep the furnace.
Should I round up to a bigger furnace to be safe?
Only to the next available size. Furnaces that are much too large heat in short bursts, which can cause uneven temperatures and extra wear. Pick the smallest model whose rated output meets the load, and let a contractor confirm it.
Estimates only. Verify quantities with your supplier before purchasing.