Furnace Sizing Calculator
A furnace is sized by the heat output your home needs, then by the input rating once you account for efficiency. This calculator estimates the required output by multiplying floor area by a heating factor in BTU per square foot that you set for your climate and insulation. It then divides the output by the furnace AFUE efficiency to give the input (nameplate) BTU rating. Because the heating factor depends on local conditions, it is a user-editable input. Treat the result as a screening figure: a full ACCA Manual J load calculation is the correct basis for selecting a furnace.
Furnace sizing formula
Required heat output = home area * heating factor
Furnace input rating = required output / (AFUE / 100)
Energy lost = furnace input rating - required output
You size the furnace by the heat output your home needs. The input rating is higher than the output because AFUE is below 100%: the energy lost is the difference between fuel consumed and heat delivered.
Notes on furnace sizing
- Set the heating factor for your climate and insulation; it is not a fixed value.
- Higher AFUE means a lower input rating delivers the same heat output.
- This is a screening estimate, not a substitute for an ACCA Manual J load calculation.
- Avoid oversizing: a too-large furnace short cycles and wastes fuel.
- The Department of Energy sets minimum AFUE standards for furnaces sold in the US.
Furnace sizing: frequently asked questions
What size furnace do I need?
Furnace size is set by your home's heating load. A quick screen multiplies floor area by a heating factor in BTU per square foot to estimate the required heat output, then divides by the furnace's AFUE efficiency to find the input rating. For accurate sizing, use an ACCA Manual J load calculation.
What is AFUE?
AFUE stands for Annual Fuel Utilization Efficiency. It is the percentage of fuel energy a furnace converts to useful heat over a year. An 80% AFUE furnace delivers 80,000 BTU of heat for every 100,000 BTU of fuel input. The U.S. Department of Energy sets minimum AFUE standards for furnaces.
What is the difference between input and output BTU?
Input BTU is the rate of fuel energy the furnace consumes. Output BTU is the heat actually delivered to the home. Output equals input times AFUE. You size the furnace by required output, then divide by AFUE to find the input rating shown on the nameplate.
What heating factor should I use?
Heating factors range roughly from 20 BTU per square foot in mild climates with good insulation to 60 or more in cold climates or leaky homes. Because it depends on your specific situation, the factor is a user-editable input here, not a fixed value. A Manual J calculation determines it precisely.
Why not just buy the biggest furnace?
An oversized furnace short cycles: it heats quickly, shuts off, then restarts often. This wastes fuel, stresses components, and creates uneven temperatures. Matching the furnace output to the calculated heating load gives steadier comfort and better efficiency than oversizing.
Official sources
- U.S. Department of Energy, furnaces and boilers: energy.gov.
- Air Conditioning Contractors of America (Manual J load calculation standard): acca.org.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.