Air-Fuel Ratio Calculator
Air-fuel ratio, lambda and equivalence ratio for any fuel — with the stoichiometric point and whether the mixture is rich or lean.
The stoichiometric AFR is the mass of air needed to burn a unit mass of fuel completely: 14.
How the air-fuel ratio calculator works
The stoichiometric AFR is the mass of air needed to burn a unit mass of fuel completely: 14.7:1 for petrol, 14.5 for diesel, 9.0 for ethanol.
Lambda is the actual AFR divided by the stoichiometric one. Lambda 1 is exactly stoichiometric; below 1 is rich (excess fuel, more power, more carbon monoxide); above 1 is lean (excess air, better economy, hotter combustion and more nitrogen oxides).
Formula: λ = AFR_actual / AFR_stoich; φ = 1/λ
Worked examples
| Inputs | Lambda | Note |
|---|---|---|
| Petrol at 13.5:1 | 0.91837 | λ 0.918 — rich, typical at full throttle |
| Stoichiometric petrol | 1 | λ exactly 1 |
| E85 | 1 | a much lower AFR for the same lambda |
FAQFrequently asked questions
What is the stoichiometric air-fuel ratio?
14.7:1 by mass for petrol — 14.7 grams of air to burn one gram of fuel completely.
What is lambda?
The actual AFR divided by the stoichiometric one. Lambda 1 is exactly stoichiometric; below is rich, above is lean.
Why do engines run rich at full throttle?
Extra fuel absorbs heat as it vaporises, cooling the charge and protecting the pistons and turbo. It also makes marginally more power.
Why does E85 have a lower AFR?
Because ethanol already contains oxygen, so less air is needed. The energy per unit of *mixture* is similar — which is why fuel flow rises about 30%.
What is the equivalence ratio?
φ = 1/λ. Combustion research prefers it because rich mixtures then give φ > 1, which reads more naturally.
Where these figures come from
- IUPAC — Standard atomic weights (2021 conventional values) — the molar-mass table
- NIST — CODATA 2018 fundamental physical constants — Avogadro constant, gas constant, speed of light
- NIST Chemistry WebBook — thermochemical data
- National Institute of Standards and Technology — the US measurement and reference-data authority
Last checked: September 2026. Atomic masses are the IUPAC conventional values; constants are CODATA 2018; equations are the standard textbook forms.