Air-Fuel Ratio (AFR) Calculator
Find the air-fuel ratio and lambda (λ) for your engine or combustion process. Enter the air and fuel masses, choose the fuel type, and see whether the mixture is rich, lean or perfectly stoichiometric.
g
g
Fuel type
Stoichiometric AFR for selected fuel: 14.70:1
- 1
Air-Fuel Ratio (AFR)
147 g ÷ 10 g = 14.7 - 2
Stoichiometric AFR for fuel
14.7The ideal AFR at which all fuel and oxygen are consumed (no excess of either). - 3
Lambda (λ)
14.7 ÷ 14.7 = 1
How does this calculator work?
AFR = mass of air ÷ mass of fuel. Lambda (λ) = AFR ÷ stoichiometric AFR for the fuel (14.7 for gasoline). λ < 1 is a rich mixture (excess fuel), λ > 1 is lean (excess air), λ = 1 is ideal for a catalytic converter. Enter air mass, fuel mass and fuel type to get the result.
Formula
How this is calculated
The air-fuel ratio (AFR) is the ratio of the mass of air to the mass of fuel entering the combustion chamber. Every fuel has a stoichiometric AFR — the exact ratio at which all fuel is burned and all oxygen consumed, with nothing left over. For gasoline this is approximately 14.7:1, meaning 14.7 kg of air per kg of fuel. Diesel sits at about 14.5:1, while alcohol fuels such as E85 need far less air (≈ 9.765:1) because they carry oxygen in their own molecules.
Lambda (λ) normalises the measured AFR against the stoichiometric AFR for the fuel in use: λ = 1 is a perfect stoichiometric mixture, λ < 1 is a rich mixture (excess fuel), and λ > 1 is a lean mixture (excess air). Three-way catalytic converters achieve peak efficiency only near λ = 1, which is why modern ECUs continuously dither fuel delivery around this point. The equivalence ratio φ = 1/λ is the same information expressed inversely — φ > 1 means rich, φ < 1 means lean.
This calculator assumes mass-based inputs. If you have volumetric flow rates (such as from an air-flow sensor in m³/s and an injector in ml/min), convert to grams first using air density ≈ 1.225 kg/m³ at standard conditions and the fuel density for your fuel type.
Frequently asked questions
Lambda compares the actual AFR to the stoichiometric ideal for that fuel. λ = 1 is a chemically perfect burn. λ < 1 (rich) provides extra fuel for cooling and maximum power but raises CO and fuel consumption. λ > 1 (lean) improves economy and CO₂ output but can cause knocking, elevated NOₓ, or misfires at extremes.
Each fuel has a different chemical formula and hydrogen/carbon/oxygen content. Hydrogen burns with only oxygen and yields only water, requiring a very high AFR of 34.3:1. Alcohols like methanol (6.475:1) contain their own oxygen atoms, reducing how much air is needed. Gasoline and diesel are broadly similar hydrocarbons at 14.5–14.7:1.
Peak power for gasoline typically occurs around AFR 12.5–13.0 (λ ≈ 0.85–0.88, rich). Best fuel economy appears near or slightly lean of stoichiometric, around AFR 15–15.5. OBD systems actively vary fuelling around λ = 1 to satisfy the catalytic converter while balancing these goals.
TG we-Calculate Editorial Team. (2026). Air-Fuel Ratio (AFR) Calculator [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/air-fuel-ratio-afr-calculator
TG we-Calculate Editorial Team. "Air-Fuel Ratio (AFR) Calculator." TG we-Calculate. 2026. https://we-calculate.com/calculator/air-fuel-ratio-afr-calculator.
TG we-Calculate Editorial Team, "Air-Fuel Ratio (AFR) Calculator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/air-fuel-ratio-afr-calculator
@misc{wecalculate_air_fuel_ratio_afr_calculator, title = {Air-Fuel Ratio (AFR) Calculator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/air-fuel-ratio-afr-calculator}}, year = {2026}, note = {TG we-Calculate} }
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