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Air-Fuel Ratio Calculator

Enter a lambda value and choose your fuel.

How it is calculated

The air-fuel ratio is the lambda value times the stoichiometric ratio of the chosen fuel: AFR = λ × AFRstoich. The tool uses these stoichiometric values (air to fuel by mass): gasoline 14.7, diesel 14.5, ethanol 9.0, methanol 6.4 and LPG 15.6. An unrecognised fuel falls back to gasoline.

Lambda is a plain ratio with no unit, and it must be above zero. The mixture is labelled Rich when λ is below 1, Lean when it is above 1 and Stoichiometric only when it is exactly 1, so λ = 0.999 already shows as Rich. The tool does not test whether a mixture is safe or whether it is inside the engine's useful range. AFR and the stoichiometric ratio are shown with two decimals.

Worked example

A tuner reads λ = 0.85 from a wideband sensor on a petrol engine. Stoichiometric AFR for gasoline is 14.70:1. AFR = 0.85 × 14.7 = 12.495, shown as 12.50:1. Because λ is below 1, the mixture is Rich, with more fuel than needed for complete burning. The same λ = 0.85 on an ethanol tune would be 0.85 × 9.0 = 7.65:1, which shows why a gasoline AFR number cannot be read for other fuels.

Try this example →

Common mistakes

  • Comparing AFR numbers across fuels. A 12.5:1 mixture is rich on gasoline but extremely lean on methanol; compare lambda instead.
  • Leaving the fuel on gasoline for an ethanol or diesel engine. The AFR then comes out wrong by the ratio of the two stoichiometric values.
  • Entering the AFR (14.7) in the lambda field. The field wants lambda, so 14.7 would be read as a very lean λ of 14.7 and give an AFR over 200.
  • Mixing up rich and lean: λ below 1 is rich (extra fuel), above 1 is lean (extra air).
  • Reading a rich or lean label as a pass or fail for the engine. Target values depend on load, fuel and tune.

About this calculator

The air-fuel ratio (AFR) is the mass of air consumed per unit mass of fuel burned in an engine, and it's usually expressed relative to lambda (λ), the ratio of the actual AFR to the fuel's stoichiometric (chemically ideal) AFR — for gasoline, stoichiometric AFR is about 14.7:1, so λ=1 means a perfectly balanced mixture. This calculator multiplies your lambda reading by the selected fuel's stoichiometric ratio to get the actual AFR, and reports whether the mixture is rich (λ below 1, more fuel than needed), lean (λ above 1, more air than needed), or right at stoichiometric.

AFR and lambda are core diagnostic numbers for automotive technicians, engine tuners and motorsport engineers, who read lambda live from a wideband oxygen sensor while tuning fuel injection maps, since running too lean risks engine-damaging detonation while running too rich wastes fuel and fouls spark plugs. The same ratio matters for alternative fuels too — ethanol, diesel and propane each have a different stoichiometric AFR, which is why the fuel selection changes the target ratio in this calculator.

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Comments (1)

  • Editorial team

    Lambda is the number that travels between fuels; AFR does not. At lambda 1 ethanol reads 9.00:1, because that is its stoichiometric ratio, while gasoline's is 14.70:1. Diesel engines normally run lean overall, so a lean label is no fault there: lambda 1.1 on diesel gives 15.95:1. Pump fuel blended with ethanol sits between the two pure fuels, and no blend is listed. Which fuel or blend would you like added?

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