Stoichiometric air-fuel ratio calculator

The stoichiometric air-to-fuel mass ratio is the exact kilograms of dry air needed to burn one kilogram of fuel with no leftover oxygen and no leftover fuel. For a hydrocarbon formula unit CxHy, complete combustion fixes the oxygen demand at C plus H over four moles of O2 per mole of fuel; dry air supplies that oxygen at about twenty-one percent by volume, so the air moles scale by one over zero point two one and convert to mass with the mean molar mass of air. Dividing air mass by fuel molar mass yields the classical AFR engineers quote as about seventeen for methane and about fifteen for typical gasoline surrogates. Students meet the derivation in combustion and thermo courses, engine calibrators use it as the lambda-equals-one anchor, and process models need a closed form free of empirical maps. This calculator accepts non-negative carbon and hydrogen atom counts, applies fixed conventional atomic weights and dry-air constants, rejects the undefined case when both counts are zero, and returns the air-fuel mass ratio together with oxygen demand, product moles, fuel formula, and the defining method label.

How carbon and hydrogen counts become a stoichiometric AFR

Start from a pure hydrocarbon written as CxHy, where C is the carbon atom count and H is the hydrogen atom count of one formula unit of fuel. Complete combustion in oxygen follows CxHy plus n_O2 of O2 goes to C of CO2 plus H over two of H2O, with the oxygen balance forcing n_O2 equals C plus H over four. That single scalar is the stoichiometric oxygen demand per mole of fuel.

Real engines and burners do not feed pure oxygen; they feed dry air that is approximately twenty-one percent O2 by volume and therefore about four point seven six moles of air for every mole of O2. Multiplying n_O2 by that air factor and by the conventional mean molar mass of dry air, typically twenty-eight point nine seven grams per mole, produces the mass of air required per mole of fuel.

The fuel mass per mole is C times the atomic mass of carbon plus H times the atomic mass of hydrogen, using the same fixed conventional weights as other KIT chemistry calculators so classroom goldens stay aligned. The stoichiometric air-to-fuel mass ratio is then mass of air divided by mass of fuel; the reciprocal fuel-to-air ratio and the pure oxygen-to-fuel mass ratio are reported alongside for convenience.

Lambda and the equivalence ratio are both one by definition at stoichiometry, because the calculator targets complete combustion with no excess air and no fuel-rich leftover. Negative atom counts are rejected immediately because a chemical formula cannot carry fewer than zero atoms of an element. Fractional counts are allowed when you model an average formula unit for a blend, for example a gasoline surrogate near C8H15, as long as at least one element contributes positive mass.

Where stoichiometric AFR calculators show up in real work

Secondary-school and university combustion courses assign stoichiometric air-fuel ratio drills after balancing CxHy plus oxygen reactions, asking students to convert atom counts into a mass ratio and to compare methane near seventeen against heavier alkanes nearer fifteen. Automotive and aerospace labs use the stoichiometric AFR as the lambda-equals-one reference when calibrating oxygen sensors, setting idle maps, or checking that a surrogate fuel formula produces a plausible air demand before dyno time.

Process and energy engineers sizing burners, boilers, or gas turbines often start from a design fuel formula and need a fast closed-form air mass before introducing humidity, flue-gas recirculation, or multi-component blend tables. If you later need mole-ratio stoichiometry between arbitrary reactants, combustion analysis from CO2 and H2O product masses, or fuel-economy unit conversion, treat those as separate search intents and landings rather than overloading this stoichiometric air-fuel ratio calculator page.