What does lambda 1.00 actually mean?

Exactly enough air to burn all the fuel, with nothing left over on either side. It is the ratio a catalytic converter needs to work on both oxides of nitrogen and unburnt hydrocarbons, which is why closed-loop cruise sits there.

What AFR should I target at full throttle?

Around lambda 0.85 to 0.88 for a naturally aspirated gasoline engine and 0.75 to 0.82 for a boosted one — richer mixtures cool the charge and pull knock away. On gasoline that is roughly 12.5:1 and 11.8:1.

Why is the stoichiometric ratio for E85 so much lower?

Ethanol carries its own oxygen, so each kilogram of fuel needs less air to burn completely. That is also why E85 needs roughly 30% more fuel flow for the same power, and why injectors often have to be resized.

Is the equivalence ratio the same as lambda?

It is the inverse. Phi = 1/lambda, so phi 1.2 and lambda 0.833 describe the same rich mixture. Engineering papers usually quote phi; engine calibration tools usually quote lambda.

Can a narrowband oxygen sensor give me these numbers?

No. A narrowband only reports whether the mixture is on the rich or lean side of stoichiometric — it has no useful resolution beyond that. Any real AFR or lambda figure comes from a wideband sensor.

Air-Fuel Ratio Calculator

Convert between AFR, lambda and equivalence ratio for gasoline, E85, diesel, methanol and more.

The Air-Fuel Ratio Calculator runs entirely in your browser. Sensor readings and tuning targets stay on your device.

Check compression ratio

About Air-Fuel Ratio Calculator

An air-fuel ratio only means something once you know what is in the tank. Gasoline burns stoichiometrically at 14.7:1 by mass, E85 at about 9.8:1 and methanol at 6.4:1, so the same 12.5:1 reading is rich on one fuel and dangerously lean on another. Lambda solves that by dividing the measured ratio by the stoichiometric one, giving a number that means the same thing everywhere: 1.00 is stoichiometric, below is rich, above is lean. This calculator converts freely between AFR, lambda and the equivalence ratio phi used in combustion work, for ten fuels plus any ethanol blend you name.

Features

Converts AFR, lambda and equivalence ratio in any direction
Ten fuels covered, from gasoline and diesel to methanol, LPG, CNG and hydrogen
Stoichiometric ratio for any ethanol blend, interpolated by percentage
Rich, lean or stoichiometric verdict with the deviation as a percentage
Common tuning targets shown as both lambda and AFR for the chosen fuel
One lambda reading translated into an AFR for every fuel in the table
Copyable summary for a tuning log

How to use the Air-Fuel Ratio Calculator

Pick whether you are starting from AFR, lambda or phi
Enter the reading from your wideband or logging software
Choose the fuel, or type an ethanol percentage for a custom blend
Read the converted values and how far the mixture sits from stoichiometric

Example

Input

AFR 12.5 on gasoline

Output

Lambda 0.850 · phi 1.176 · 15% rich of stoichiometric

The same lambda on E85 would be an AFR of 8.30, which is why tuners work in lambda.

Common errors & troubleshooting

The AFR from a wideband looks wrong after switching to E85. — Most gauges display AFR against a fixed gasoline stoichiometric value. The sensor measures lambda; convert with the E85 figure and the number will make sense again.
Lambda and phi seem to be moving in opposite directions. — They are reciprocals by definition. Lambda above 1 is lean, and phi above 1 is rich — combustion literature uses phi, engine tuning usually uses lambda.
The stoichiometric figure for the pump fuel does not match the spec sheet. — Pump gasoline already contains ethanol in most markets. E10 sits near 14.1:1 rather than the 14.7:1 quoted for pure gasoline — use the ethanol percentage field for the fuel you actually have.
A diesel reading looks impossibly lean. — That is normal. Diesels run unthrottled and idle at lambda 5 or more; they are load-controlled by fuelling, so a lean number is not the warning sign it would be on a spark-ignition engine.

Frequently asked questions

What does lambda 1.00 actually mean?: Exactly enough air to burn all the fuel, with nothing left over on either side. It is the ratio a catalytic converter needs to work on both oxides of nitrogen and unburnt hydrocarbons, which is why closed-loop cruise sits there.
What AFR should I target at full throttle?: Around lambda 0.85 to 0.88 for a naturally aspirated gasoline engine and 0.75 to 0.82 for a boosted one — richer mixtures cool the charge and pull knock away. On gasoline that is roughly 12.5:1 and 11.8:1.
Why is the stoichiometric ratio for E85 so much lower?: Ethanol carries its own oxygen, so each kilogram of fuel needs less air to burn completely. That is also why E85 needs roughly 30% more fuel flow for the same power, and why injectors often have to be resized.
Is the equivalence ratio the same as lambda?: It is the inverse. Phi = 1/lambda, so phi 1.2 and lambda 0.833 describe the same rich mixture. Engineering papers usually quote phi; engine calibration tools usually quote lambda.
Can a narrowband oxygen sensor give me these numbers?: No. A narrowband only reports whether the mixture is on the rich or lean side of stoichiometric — it has no useful resolution beyond that. Any real AFR or lambda figure comes from a wideband sensor.

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