Furnace Sizing Calculator
A furnace must produce enough heat to cover a home's design heat loss without being grossly oversized. A simple first-pass estimate multiplies the conditioned floor area by a climate heating factor in BTU per square foot, which rises with how cold your region gets. This calculator estimates the required heating output, then converts it to the furnace input rating using the unit's AFUE efficiency. For a final selection, a Manual J load calculation is recommended.
Furnace sizing formula
Required output BTU = floor area * heating factor
Furnace input rating = output BTU / (AFUE / 100)
Output kW = output BTU / 3,412
Heating factor depends on climate: roughly 30 to 35 BTU per sq ft in mild zones, 40 to 45 in moderate zones, and 50 to 60 in cold zones. AFUE (Annual Fuel Utilization Efficiency) is the share of fuel converted to usable heat; a 95 percent AFUE furnace needs an input rating higher than its output.
Worked example
A 2,000 square foot home in a moderate zone uses a 40 BTU per sq ft factor: required output = 2,000 times 40 = 80,000 BTU per hour. With a 95 percent AFUE furnace, the input rating must be 80,000 / 0.95 = 84,210 BTU per hour, so an 84,000 to 88,000 BTU input furnace fits.
Furnace sizing notes
- Climate heating factors: about 30 to 35 BTU per sq ft in warm zones, 40 to 45 in moderate zones, 50 to 60 in cold zones.
- AFUE measures seasonal efficiency; the U.S. minimum for new non-weatherized gas furnaces is 80 percent, with high-efficiency models reaching 90 to 98 percent.
- This square-foot method is a rough estimate; an ACCA Manual J calculation accounts for insulation, windows, and air leakage.
- Oversized furnaces short-cycle, reducing comfort and efficiency, so do not round up aggressively.
Furnace Sizing Calculator: frequently asked questions
How many BTU furnace do I need for 2,000 square feet?
It depends on climate. Using 40 BTU per square foot for a moderate zone, a 2,000 square foot home needs about 80,000 BTU per hour of heating output. Colder zones may need 50 to 60 BTU per square foot, and milder zones less.
What is AFUE?
AFUE stands for Annual Fuel Utilization Efficiency. It is the percentage of fuel energy a furnace turns into usable heat over a season. A 95 percent AFUE furnace delivers 95 BTU of heat for every 100 BTU of fuel; the rest is lost to exhaust.
What is the difference between input and output BTU?
Input BTU is how much fuel energy the furnace consumes per hour; output BTU is the usable heat delivered. Output equals input times AFUE. To deliver a target output, divide it by AFUE to find the input rating to shop for.
Should I oversize my furnace to be safe?
No. An oversized furnace heats the home quickly then shuts off, cycling on and off frequently. This reduces comfort, wears components, and lowers efficiency. Size to the calculated load, and use a Manual J calculation for the final decision.
Sources and methodology
- U.S. Department of Energy: Furnaces and Boilers (AFUE efficiency).
- The square-foot heating-factor method and 3,412 BTU per kWh conversion are standard HVAC engineering references; factors are user-editable inputs.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.