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 |
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.