Formula & worked example
Input=Load·(1+margin)/η.
A 60,000 BTU/h design load with 10% margin and 92% efficiency needs about 71,739 BTU/h input rating.
Heating & HVAC engineering calculator · Updated Sep 17, 2026
Furnace Size Calculator — Heating Load, Efficiency & Input BTU/h
Convert a design heating load into required furnace input capacity from AFUE/thermal efficiency and sizing margin.
Inputs
Live result
Required delivered heat 66,000 BTU/h (19.342691 kW)
Minimum input rating 71,739.13 BTU/h
Input rating 71.73913 kBTU/h
Assumptions & limits
- Load should come from a defensible heat-loss calculation rather than floor area alone.
- Cycling, staging and equipment derating require manufacturer/design review.
Frequently asked questions
What does this Furnace Size Calculator calculate?
Convert a design heating load into required furnace input capacity from AFUE/thermal efficiency and sizing margin.
What units are supported?
Inputs state their unit basis and results include practical SI and US customary equivalents where useful, so the calculation can be checked against field data and equipment schedules.
Can I use this for final equipment selection?
Use it for transparent first-pass sizing and engineering cross-checks. Final selections should use measured conditions, manufacturer performance data and applicable codes or design standards.