This AFR calculator converts between air-fuel ratio and lambda (λ) for the common fuels — gasoline, diesel, E85, ethanol and methanol — using each fuel’s stoichiometric ratio, the chemically exact mixture where fuel and oxygen balance perfectly.

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Two Dialects for One Mixture

Air-fuel ratio states the mixture as a mass proportion — 14.7 kg of air per 1 kg of gasoline at the stoichiometric point — while lambda normalizes it: λ = actual AFR ÷ stoichiometric AFR, making 1.00 the universal “exactly right” on any fuel. The defaults show the translation tuners do at the dyno: gasoline at AFR 12.5 is λ = 12.5/14.7 = 0.85, a classic power-rich mixture; and λ 1.10 (a lean cruise) on gasoline is AFR = 16.17. The fuel presets carry the standard stoichiometric values — gasoline 14.7, diesel 14.5, E85 ≈ 9.8 (varies with the actual ethanol fraction, 9.7–10.0 across the seasonal blends), ethanol 9.0, methanol 6.45 — the alcohol fuels needing proportionally more fuel mass because they arrive carrying part of their own oxygen.

Why engines rarely run at λ 1.00 despite the chemistry calling it exact: gasoline engines visit stoichiometry at cruise because the three-way catalyst only works in a narrow window around λ = 1 (it must oxidize and reduce simultaneously), enrich to λ ≈ 0.85 at full power (evaporative cooling protects parts and suppresses knock), and diesels run permanently lean (λ 1.3–2.0) because their combustion is mixing-controlled rather than premixed. Wideband oxygen sensors report lambda natively — the AFR on a gauge is usually the sensor’s lambda multiplied by a fuel constant the gauge assumed, which is why switching to E85 on a gasoline-calibrated gauge reads nonsense until the constant is changed.

A dyno-room habit worth copying: log lambda, not AFR, in your tuning notes. Lambda entries survive fuel changes intact (a λ 0.85 target means the same enrichment on gasoline, E85 or methanol), while an AFR notebook page from a gasoline season silently poisons the first E85 session — the units look identical and only the stoich denominator changed, which is precisely the failure mode this calculator’s two-way view is built to expose.

Worked Examples (Verified)

  • Power tune (defaults): gasoline AFR 12.5 → λ = 0.85.
  • Lean cruise: gasoline λ 1.10 → AFR = 16.17.
  • Same gauge, different fuel: AFR 9.8 on E85 = λ 1.00 — stoichiometric, not “crazy rich.”

AFR Discipline

  • Never quote AFR without the fuel: the number is meaningless across fuels; lambda is the portable currency.
  • E85’s stoich drifts with blend (winter/summer ethanol content): flex-fuel sensors exist precisely because 9.8 is an estimate, not a constant.
  • Wideband gauges convert λ → AFR with an assumed fuel constant; verify the setting after any fuel change.
  • Rich/lean are relative to YOUR fuel’s stoich: diesel at AFR 18 is lean-and-normal; gasoline at 18 is misfire territory.
  • Mass basis, always: AFR is kilograms (or pounds) of air per unit fuel mass — volumetric ratios differ by fuel density and are not what’s printed.

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Frequently Asked Questions

How do you convert AFR to lambda?

λ = AFR ÷ stoichiometric AFR for the fuel in use; and AFR = λ × stoichiometric AFR. Gasoline (stoich 14.7): AFR 12.5 → λ 0.85; λ 1.10 → AFR 16.17. This calculator carries the standard stoich values for gasoline, diesel, E85, ethanol and methanol and converts both directions at once.

What is the stoichiometric AFR for common fuels?

Gasoline 14.7:1, diesel 14.5:1, propane ≈ 15.5:1, natural gas ≈ 17.2:1, E85 ≈ 9.8:1, ethanol 9.0:1, methanol 6.45:1 (mass air per mass fuel). Alcohols sit lower because their molecules carry oxygen — part of the oxidizer arrives inside the fuel, so less air is needed per kilogram. Hydrogen is the extreme at ≈ 34:1 (its mass is so low that the air dominates the ratio).

What AFR makes the most power?

For gasoline, maximum power typically occurs rich of stoich at λ ≈ 0.82–0.88 (AFR ≈ 12.1–12.9): the excess fuel’s evaporation cools the charge, and the extra fuel ensures every oxygen molecule finds a partner. Best economy sits slightly lean (λ 1.05–1.15 where combustion stability allows). Diesels make power by adding fuel toward the smoke limit while remaining overall lean — a different regime entirely, governed by mixing rather than a premixed ratio.

Why does my wideband show AFR 14.7 on E85 at cruise?

Because most gauges display lambda × a fuel constant, defaulting to gasoline’s 14.7: at λ 1.00 the display reads “14.7” regardless of the fuel actually burning. On E85 your true mass AFR at λ 1 is 9.8 — the gauge is speaking lambda in gasoline costume. Switch the gauge’s fuel setting (or read lambda directly) before tuning, or every target will be off by the stoich ratio.

Rich vs lean — what do the terms mean exactly?

Relative to stoichiometric for that fuel: rich = more fuel than stoich (λ < 1, AFR below the fuel's stoich), lean = less fuel (λ > 1). Rich mixtures run cooler in the chamber, waste fuel, and produce CO/HC; lean mixtures risk knock, high NOx and (in gasoline engines) misfire past about λ 1.25–1.3 in conventional designs. The catalyst window pins production gasoline cars to λ 1.00 ± a hair at cruise regardless.

How is AFR measured?

By a wideband oxygen sensor in the exhaust: it measures residual oxygen and infers the mixture the engine actually burned, reported as lambda (converted to AFR for display). Narrowband sensors only bracket λ = 1 (they switch around stoich) — adequate for catalyst control, useless for tuning. Accurate AFR work also weighs fuel and air directly on engine dynos (the primary standard the sensors are checked against).

Does altitude change the target AFR?

The TARGET ratio doesn’t change (stoichiometry is chemistry), but the air mass per intake stroke falls with altitude, so fuel delivery must fall proportionally to hold the same AFR — the job carburetors did badly (they run rich climbing) and EFI does automatically via air-mass measurement. Turbocharged engines compensate boost-side. Lambda feedback keeps the realized ratio on target wherever the hardware can meter finely enough.

Is this AFR calculator free?

Yes — free, instant, no sign-up. Pick the fuel, enter AFR or lambda, and read both directions of the conversion.