Engineering calculator · Updated 17 September 2026
Boiler Excess Air from O₂ Calculator — Flue-Gas O₂ & Air Ratio
Estimate excess combustion air from dry flue-gas oxygen using the common 21/(21−O₂) air-ratio approximation. Returns excess-air percent, air ratio and sensitivity.
Inputs
Live results
- Air ratio λ 1.1666667
- Excess air 16.666667%
- Theoretical-air share 85.714286% of actual air
Formula
λ=21/(21−O2); excess air=(λ−1)×100%.
Worked example
3% dry O₂ corresponds to about 16.7% excess air with this screening relation.
Assumptions and limits
- Use dry flue-gas O₂ below 21%.
- Fuel composition, leakage and analyzer location can require a fuel-specific combustion calculation.
FAQs
What does this Boiler Excess Air from O₂ Calculator calculate?
Estimate excess combustion air from dry flue-gas oxygen using the common 21/(21−O₂) air-ratio approximation. Returns excess-air percent, air ratio and sensitivity.
Is this suitable for final equipment selection?
Use the result for engineering screening and operating diagnostics. Confirm final selections against measured data, OEM curves and applicable codes.
Why are the assumptions explicit?
Plant results depend on reference conditions, pressure basis, efficiencies and measurement locations. Showing assumptions keeps the calculation auditable.