Calculate stoichiometric air–fuel ratio for a C/H/O fuel
The AFR calculator accepts a fuel formula containing carbon, hydrogen, and optional oxygen, then estimates the dry-air mass required for complete stoichiometric combustion to carbon dioxide and water. Enter a formula such as CH4, C8H18, or C2H6O and the actual air-to-fuel mass ratio. The result reports stoichiometric AFR, lambda, equivalence ratio, and a lean/rich guide.
Air–fuel ratio is mass of air divided by mass of fuel. A displayed value of 14.7:1 means 14.7 mass units of air per one mass unit of fuel. Lambda compares actual AFR with stoichiometric AFR, while equivalence ratio phi is its reciprocal. Naming both ratios prevents the common mistake of calling a lean mixture rich because the reciprocal convention was assumed.
Balance oxygen for complete combustion
For CxHyOz, complete combustion requires x + y/4 − z/2 moles of O2 per mole of fuel. Carbon forms CO2 and hydrogen forms H2O; oxygen already present in the fuel reduces external oxygen demand. The calculator rejects formulas outside carbon, hydrogen, and oxygen because nitrogen, sulfur, metals, ash, water, and blended composition require additional product assumptions.
The required oxygen is divided by 0.2095, the stated dry-air oxygen mole fraction, to estimate moles of dry air. Multiplying by a dry-air molar mass of 28.965 g/mol gives air mass, which is divided by fuel molar mass. Every intermediate value is displayed so the oxygen balance and mass basis can be checked independently.
Read lambda and equivalence ratio
Lambda equals actual AFR divided by stoichiometric AFR. Lambda above one indicates more air than the stoichiometric basis and is labelled lean; below one indicates less air and is labelled rich. Equivalence ratio phi equals stoichiometric AFR divided by actual AFR, or 1/lambda, so its rich and lean directions are reversed. Near one means near the selected stoichiometric model.
These ratios describe the entered bulk mixture, not combustion completeness or emissions by themselves. Ignition, mixing, residual exhaust, humidity, pressure, temperature, dissociation, wall films, and transient operation can keep real products from the ideal CO2-and-water endpoint. A sensor lambda value and a commanded fuel ratio also require the correct fuel calibration.
Use the correct fuel formula and basis
A pure hydrocarbon formula can be entered directly. Oxygenated fuels may include O, but blends such as commercial gasoline, E10, biodiesel, natural gas, or producer gas do not have one universal molecular formula. Use a measured elemental analysis or an approved representative composition when engineering accuracy matters. Do not invent one formula from a product name.
The actual AFR must be positive and use the same mass basis as the result. The formula parser expands subscripts and groups but rejects unsupported elements and noncombustible C/H requirements. Compare the stoichiometric result with a trusted fuel property source before using it in engine calibration, burner control, emissions analysis, or any safety-related decision.
Engineering limits and private estimation
This calculator is an idealized educational balance. It does not model dissociation, incomplete combustion, adiabatic flame temperature, heating value, detonation, flammability limits, exhaust composition, moisture, or equipment-specific control. Fuel handling and combustion systems require qualified procedures, instrumentation, ventilation, and applicable safety standards.
The values are calculated in the current browser tab. NexaCurrent does not upload the quantities, formulas, chemical names, element choices, or other information entered here, and no account is needed. Copying is a deliberate action after the answer and working are visible. Refreshing or leaving the page clears the current calculation, so keep a copied result if it is needed for later study.