Air-Fuel Ratio Calculator
Converts between AFR and lambda for petrol, E10, E85, diesel, methanol and LPG, and explains what a reading means at the load you are at.
Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.
The formula
λ = AFR ÷ stoichiometric AFR stoichiometric: petrol 14.7, E10 14.1, E85 9.8, diesel 14.5, methanol 6.4
Stoichiometric is the ratio at which every fuel molecule meets exactly enough oxygen. Lambda divides the actual ratio by that figure, so it is 1.0 at stoichiometric for any fuel — which makes it the honest way to compare mixtures across fuels.
What to watch out for
Target mixtures by situation, in lambda:
- Cruise and idle: 1.00. The catalyst needs it and economy is best near it.
- Naturally aspirated full throttle: 0.88–0.92 for best power.
- Boosted full throttle: 0.78–0.85. The extra fuel is there to cool the charge and hold off detonation, not for power.
- Lean at load — above 1.05 — is where pistons melt. A lean misfire at idle is harmless; a lean mixture at 6,000 rpm on boost is not.
A gauge marked in AFR is calibrated for petrol. On E85 it will show 12.5 when the real AFR is 8.3 — the lambda is what it is actually measuring.
Frequently Asked Questions
What is a stoichiometric air-fuel ratio?
The ratio at which the fuel burns completely with no oxygen left over — 14.7:1 by mass for petrol, 14.1 for E10, 9.8 for E85 and 14.5 for diesel. Lambda is 1.0 at stoichiometric for every fuel.
What AFR is best for power?
Around lambda 0.88–0.92 — 12.9 to 13.5:1 on petrol — for a naturally aspirated engine. Boosted engines run richer, 0.78–0.85, to keep the charge cool and avoid detonation.
What is the difference between AFR and lambda?
AFR is the mass ratio of air to fuel and depends on the fuel. Lambda is AFR divided by the stoichiometric AFR, so it is 1.0 at stoichiometric whatever the fuel. A lambda of 0.85 is 12.5:1 on petrol but 8.3:1 on E85.