Air-Fuel Ratio Calculator

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.