How to use this estimate
- Output capacity is the heat delivered; input capacity is the nameplate fuel input.
- Compare the estimate with a contractor’s Manual J result and the furnace’s low-stage output.
- Oversizing can create short cycles, uneven temperatures, and unnecessary noise.
Transparent method
We expose the planning formula so you can challenge it and replace assumptions with local evidence.
floor area × climate BTU/h per sq ft × envelope factor ÷ rated efficiencyWhat this calculator cannot see
- This broad method cannot see room-by-room losses, ventilation, exposed floors, or local winter design temperature.
- A licensed professional must verify combustion venting and gas supply.
Professional verification
Use this result to prepare questions and compare scenarios. Final equipment selection, electrical work, refrigerant work, combustion, duct design, and permitting should be handled by qualified professionals following local codes and manufacturer instructions.