Furnace Size Calculator
Input Rating Is Bigger Than Output for a Reason
A furnace's nameplate lists two numbers: the BTU output it actually delivers as heat, and the larger BTU input it consumes as fuel to get there. The gap between them is combustion efficiency — expressed as AFUE (Annual Fuel Utilization Efficiency) — and it's an easy detail to overlook when comparing furnace sizes across different efficiency tiers. This calculator estimates required output from the space, then works backward to the input size a furnace needs at a given AFUE.
The Formula
Furnace Input Size = Output BTU Needed / (AFUE / 100)
Climate heat factor ranges from 25 BTU/sq ft in warm climates to 50 in cold ones. Insulation multiplier adjusts for envelope quality (0.85 for good insulation, 1.2 for poor). Dividing the required output by AFUE (as a decimal) accounts for combustion losses — a 95% AFUE furnace needs to burn more input fuel than it actually delivers as heated air.
A Worked Example
| Step | Value |
|---|---|
| Output BTU Needed (1800 × 50 × 1.0 × 1.0) | 90,000 BTU/hr |
| Furnace Input Size (90,000 / 0.95) | 94,737 BTU/hr |
A lower-efficiency 80% AFUE furnace would need a 112,500 BTU/hr input rating to deliver the same 90,000 BTU/hr of heat.
Where This Calculation Matters
- Furnace replacement — matching a new furnace's input rating to the home's actual heat loss rather than simply duplicating the old unit's size.
- Efficiency tier comparison — a higher-AFUE furnace can use a smaller input rating to deliver the same output, which affects both price and gas consumption.
- New construction sizing — estimating furnace requirements before ductwork and equipment are finalized.
- Insulation upgrade payback — seeing how much smaller a furnace could be after an insulation improvement.
How to Use This Calculator
- Enter the Room/Space Area in square feet.
- Enter the Ceiling Height in feet (defaults to 8).
- Select the Climate Zone and Insulation quality.
- Enter the Furnace AFUE percentage, or leave blank to use the default of 95%.
- Select Calculate to see the recommended furnace input size.
Related Calculations
Check cooling-side sizing for the same space with the HVAC Load Calculator, or compare against a heat pump alternative using the Heat Pump Sizing Calculator.