Furnace BTU Calculator
Heating capacity by home size, climate zone, and insulation.
You need
70,000 BTU/hr
Match to furnace OUTPUT (input x AFUE). Confirm with a Manual J before buying.
- Furnace input at 80% AFUE
- 90,000 BTU/hr
- Furnace input at 95% AFUE
- 75,000 BTU/hr
Estimates only. Verify quantities before ordering.
How this is calculated
Required output = square footage x climate factor (30 to 55 BTU per sq ft from hot to subarctic climates) x insulation factor (0.9 well insulated, 1.0 average, 1.1 older or drafty), rounded to the nearest 5,000. Furnace output = input BTU x AFUE.
Enter your home's square footage, pick your climate region and insulation quality, and this calculator estimates the heating output you need in BTU per hour. The rule of thumb runs from about 30 to 35 BTU per square foot in the warm South up to 50 to 60 in the coldest northern states, adjusted for how well the house holds heat.
Furnaces are sold by input BTU and efficiency (AFUE), and what you need is output: input times AFUE. A 100,000 BTU furnace at 80% AFUE delivers 80,000 BTU of heat; a 95% unit delivers 95,000 from the same input. This estimate is for shortlisting equipment; a contractor's Manual J calculation is the real sizing document for a purchase, and oversizing causes short cycling just like with air conditioners.
Furnace sizes for common homes
Rows assume average insulation. Output BTU shown; divide by AFUE for the input rating on the furnace label.
| Home | Output BTU/hr | Input at 80% |
|---|---|---|
| 1,200 sq ft, moderate climate | 55,000 BTU/hr | 70,000 BTU/hr |
| 1,600 sq ft, moderate climate | 70,000 BTU/hr | 90,000 BTU/hr |
| 2,000 sq ft, moderate climate | 90,000 BTU/hr | 115,000 BTU/hr |
| 2,000 sq ft, cold climate | 100,000 BTU/hr | 125,000 BTU/hr |
| 2,500 sq ft, very cold climate | 140,000 BTU/hr | 175,000 BTU/hr |
Fuel type and what it means for sizing
Natural gas is the most common and usually the cheapest to run where it is available. Propane burns similarly but costs more per unit of heat and requires a tank, so operating cost matters more in the equipment decision. Oil furnaces remain common in parts of the Northeast, with higher maintenance requirements and a tank to manage.
Electric resistance furnaces are cheap to buy, essentially 100 percent efficient at converting electricity to heat, and expensive to run almost everywhere because electricity costs more per unit of heat than gas. Heat pumps are the exception, moving heat rather than generating it and delivering two to four times the heat per unit of electricity. Modern cold-climate heat pumps hold useful capacity well below freezing, which was not true of earlier generations.
Sizing works in output BTU, but furnaces are advertised by input. A unit at 80 percent AFUE delivers 80,000 BTU of output from 100,000 of input; a 96 percent condensing unit delivers 96,000 from the same input. Matching an output requirement to an input rating is the most common sizing error and produces a furnace roughly 20 percent larger than intended.
AFUE, staging, and where the comfort comes from
AFUE is annual fuel utilization efficiency, the fraction of fuel converted to usable heat over a season. The federal minimum for new gas furnaces is 80 percent, and condensing units reach 90 to 98 percent by extracting heat from the exhaust until water vapor condenses out of it. That condensate is acidic and needs a drain, and the flue changes to PVC, both of which affect installation cost.
Single-stage furnaces run at full output whenever they run. Two-stage units run at roughly 65 percent most of the time and step up in severe cold, and modulating units vary continuously. Longer, gentler cycles at partial load hold the temperature steadier, move air more consistently, and are quieter, which is where most of the perceived comfort improvement in a premium furnace actually comes from.
Variable-speed blowers are a separate feature that often accompanies staging and matter for the same reason. They also improve air filtration and summer dehumidification, since the same blower serves the air conditioner.
The ductwork usually limits the result
A furnace can only deliver what the duct system can carry. Undersized returns, long flexible runs, crushed sections, and leaks at joints all reduce airflow, and a high-efficiency furnace connected to poor ducts will underperform a basic one on good ducts. Duct leakage in unconditioned attics and crawlspaces commonly wastes 20 to 30 percent of the heat produced.
Before upgrading equipment, it is worth having the ducts inspected and sealed with mastic rather than cloth tape, and checking that return air paths exist for rooms with doors that close. A bedroom with a supply register and no return path pressurizes when the door shuts, which reduces flow into it and pulls outside air into the rest of the house.
This is also why a proper Manual J load calculation matters more than any rule of thumb, and Manual D for the duct design alongside it. The rule of thumb this calculator uses is a sound starting point for budgeting and for sanity-checking a contractor quote, but the equipment that gets installed should be sized on the actual house.
Common questions
How many BTU do I need per square foot for heating?
Roughly 30 to 35 in hot climates, 40 to 45 in moderate ones, and 50 to 60 in the coldest states, per square foot of heated space with average insulation. Insulation quality swings the answer by 10 to 20% in either direction, which is why the calculator adjusts for it.
What size furnace for a 2,000 square foot house?
In a moderate climate with average insulation, about 90,000 BTU of output, which means roughly a 110,000 BTU input furnace at 80% AFUE or 95,000 at 95% AFUE. In the cold North the same house wants 100,000 BTU or more of output.
Is a bigger furnace safer than a smaller one?
No. An oversized furnace heats the house in short blasts and shuts off, which wears components, leaves rooms uneven, and never runs long enough to circulate air properly. Size to the load; if you are between sizes, the smaller unit with good insulation usually wins.
What is AFUE?
Annual Fuel Utilization Efficiency: the fraction of fuel that becomes heat in your house rather than going up the flue. 80% is the budget standard, 90 to 98% is the condensing tier. Divide the required output by AFUE to get the input rating printed on the furnace.