Air/Fuel Ratio & Lambda Calculator
Convert between air/fuel ratio and lambda for gasoline, E85, methanol, and diesel. Includes target ranges for idle, cruise, and WOT.
Reading from your wideband O2 sensor
AFR and Lambda
Air/Fuel Ratio (AFR) is the mass ratio of air to fuel in the combustion chamber. Lambda (λ) normalizes this across all fuel types: λ = measured AFR / stoichiometric AFR. Lambda 1.0 means stoichiometric — the chemically ideal ratio where all fuel and all oxygen are consumed.
Why Lambda Matters More Than AFR
Different fuels have different stoichiometric ratios. Gasoline is 14.7:1, E85 is 9.76:1, methanol is 6.45:1. A 12:1 AFR is rich on gasoline but lean on E85. Lambda removes this confusion — λ0.85 means the same thing (rich, max power) regardless of fuel type.
Wideband vs Narrowband
Factory narrowband O2 sensors only tell you "rich" or "lean" relative to stoich. A wideband sensor gives you an actual AFR reading across the full range. If you're tuning, you need a wideband.
Target Ranges
- Idle: λ0.98–1.02 — near stoich for smooth idle and emissions
- Cruise: λ1.00–1.05 — stoich to slightly lean for fuel economy
- WOT: λ0.85–0.90 — rich for maximum power and exhaust/piston temperature protection
- Cold start: λ0.82–0.88 — extra rich to compensate for poor fuel atomization