Air Fuel Ratio Calculator

Work out air fuel ratio instantly with clear inputs, formula shown and shareable results.

About Air Fuel Ratio Calculator

Air fuel ratio is air mass over fuel mass, and lambda is that divided by the stoichiometric ratio — 14.7:1 for petrol. Lambda 1 gives the cleanest emissions for a catalyst, while full-load operation is deliberately run rich at lambda 0.85-0.9 to control combustion temperature.

Formula

Air fuel ratio

AFR = air mass / fuel mass; lambda = AFR / stoichiometric AFR

Worked example

With the default values below, Air Fuel Ratio Calculator returns Air fuel ratio of 13.33. Change any input above and every figure here is recalculated in your browser.

Inputs used

Air mass flow: 320 kg/h
Fuel mass flow: 24 kg/h
Stoichiometric ratio: 14.7 :1

Results

Air fuel ratio: 13.33 :1
Lambda: 0.9070 λ
Mixture: Rich - cooler and safer under load, poorer economy
Equivalence ratio (phi): 1.1025

Step by step

1Values used
2Air fuel ratio
3Air fuel ratio= 13.33 :1
4Lambda= 0.9070 λ
5Mixture= Rich - cooler and safer under load, poorer economy
6Equivalence ratio (phi)= 1.1025

Assumptions and limits

The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why run rich at full load?

Excess fuel evaporates and cools the charge and the exhaust valves, protecting against detonation and thermal damage to the turbine and catalyst.

Does the stoichiometric ratio vary by fuel?

Yes. It is 14.7 for petrol, 14.5 for diesel, 9.0 for ethanol and 17.2 for methane, because the carbon-hydrogen ratio differs.