Air-Fuel Ratio Calculator

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Ideal ratio for complete combustion (typically 14.7 for gasoline).
Chemistry

The Air-Fuel Ratio Calculator finds the AFR — the mass of air divided by the mass of fuel in a combustion mixture — plus the equivalence ratio (lambda) relative to the stoichiometric AFR. Lambda below 1 is a rich mixture, above 1 is lean, and 1 is ideal.

Formula

AFR = mass of air / mass of fuel; λ = AFR / stoichiometric AFR
  • AFR is the mass of air divided by the mass of fuel in the mixture.
  • The equivalence ratio lambda (λ) compares the actual AFR to the stoichiometric AFR for complete combustion.
  • λ = 1 is the ideal stoichiometric mix; λ < 1 is rich (excess fuel) and λ > 1 is lean (excess air).
  • For gasoline the stoichiometric AFR is about 14.7:1; diesel is around 14.5:1 and many other fuels differ.

Example Calculation

Inputs
  • Mass of air: 14.7 g
  • Mass of fuel: 1 g
  • Stoichiometric AFR: 14.7

With 14.7 g of air per 1 g of gasoline and a stoichiometric AFR of 14.7: AFR = 14.7 and λ = 14.7 ÷ 14.7 = 1.0, exactly the ideal stoichiometric mixture.

Frequently asked questions

What is the air-fuel ratio?
It is the mass of air divided by the mass of fuel in a combustion mixture, written for example as 14.7:1.
What does lambda (λ) mean?
Lambda is the actual AFR divided by the stoichiometric AFR. λ = 1 is ideal, λ < 1 is rich and λ > 1 is lean.
What is the stoichiometric AFR for gasoline?
About 14.7:1 by mass, meaning 14.7 grams of air for every gram of gasoline.
Is a rich or lean mixture better?
It depends on the goal: rich mixtures make more power and run cooler, while lean mixtures improve fuel economy but can raise combustion temperatures.
Does AFR change with fuel type?
Yes. Each fuel has its own stoichiometric ratio, so enter the correct stoichiometric AFR for an accurate lambda.