Air-Fuel Ratio (AFR) Calculator

Air-fuel ratio, commonly abbreviated AFR, compares the mass of air supplied to combustion with the mass of fuel. An AFR of 14.7 to 1 means 14.7 units of air mass are present for every one unit of fuel mass. Because it is a mass ratio, the same unit can be used for both entries: grams with grams, kilograms with kilograms, or pounds with pounds. The units cancel, leaving a ratio that engineers and technicians use to describe mixture composition. A stoichiometric mixture contains exactly enough oxygen, in an ideal chemical model, to oxidize all fuel without leftover oxygen or unburned fuel. Gasoline is often represented by a stoichiometric AFR near 14.7 to 1, but the correct value varies with fuel composition. Ethanol, methanol, diesel, natural gas, and blended fuels each have different stoichiometric requirements. Enter the reference appropriate to the actual fuel rather than assuming the gasoline default in every case. Lambda normalizes the measured AFR by the stoichiometric AFR. Lambda equal to one indicates a stoichiometric mixture. A lambda below one is fuel-rich because the mixture has less air per unit fuel than the reference, while lambda above one is lean. Equivalence ratio is the reciprocal of lambda, so values above one are rich and values below one are lean. Reporting normalized values makes mixtures easier to compare across fuels with different stoichiometric ratios. Actual combustion behavior depends on more than a bulk ratio. Fuel atomization, charge temperature, residual gas, cylinder distribution, transient operation, sensor placement, and combustion efficiency all matter. Wideband oxygen sensors usually infer lambda from exhaust composition rather than directly weighing incoming air and fuel. Use this calculator to convert measured mass flows, review tuning targets, or study combustion relationships, but follow manufacturer guidance and use calibrated instrumentation for engine adjustments. Excessively rich or lean operation can reduce performance, increase emissions, damage components, or create unsafe conditions.