Air Fuel Ratio Calculator
Convert an air-fuel ratio to lambda for gasoline, E85, diesel or methanol and see whether the mixture is rich or lean.
How it works
Lambda (λ) = actual AFR ÷ stoichiometric AFR (λ < 1 rich, λ = 1 stoich, λ > 1 lean)
Lambda expresses an air-fuel ratio relative to the chemically perfect (stoichiometric) mixture for that specific fuel, which lets you compare fuels on one scale. Divide the measured AFR by the fuel’s stoichiometric ratio: exactly 1.00 is stoichiometric, below 1.00 means extra fuel (rich), and above 1.00 means extra air (lean). Because each fuel has its own stoichiometric ratio — 14.7 for gasoline but only 9.8 for E85 — the raw AFR number alone is meaningless without knowing the fuel, whereas lambda is directly comparable. Wideband oxygen sensors report in lambda for exactly this reason.
Worked example
A gasoline engine reading 14.7:1 has lambda 14.7 ÷ 14.7 = 1.000 — dead stoichiometric. Read 13.2:1 on the same fuel and lambda is 13.2 ÷ 14.7 ≈ 0.898, about 10% rich, a common target under full-throttle boost for a cooler, safer burn.
Frequently asked questions
What is the difference between AFR and lambda?
AFR is the raw mass ratio of air to fuel, which differs by fuel. Lambda normalises it against that fuel’s stoichiometric ratio, so lambda 0.90 is 10% rich on any fuel while the equivalent AFR number changes from fuel to fuel.
Is rich or lean better for performance?
Under high load, engines usually run slightly rich (around lambda 0.85–0.90) for cooler combustion and knock protection. Cruising near stoichiometric (lambda 1.0) is best for efficiency and emissions. Running lean under load raises temperatures and risks damage.
Why does E85 need so much more fuel?
Ethanol contains less energy per gallon and needs far more of it per unit of air — its stoichiometric ratio is about 9.8:1 versus gasoline’s 14.7:1. That is why switching to E85 requires larger injectors or more fuel-pump capacity.
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Educational estimate. Real-world results vary with conditions, tuning and measurement.