What is the stoichiometric AFR for gasoline?

For gasoline (petrol), the stoichiometric air-fuel ratio is approximately 14.7:1, meaning 14.7 grams of air are required to completely combust 1 gram of fuel. This is also written as λ = 1.

What does lambda mean in engine tuning?

Lambda (λ) is the equivalence ratio: actual AFR divided by stoichiometric AFR. λ = 1 means perfect stoichiometric combustion. λ 1 is lean (excess air).

Is rich or lean better for performance?

Slightly rich mixtures (λ 0.85–0.95) typically produce peak power because they ensure complete oxygen consumption and help cool combustion. Lean mixtures improve fuel economy but can cause detonation and overheating under load.

What stoichiometric ratio should I use for E85?

E85 (85% ethanol, 15% gasoline) has a stoichiometric AFR of approximately 9.8:1. Pure ethanol is 9.0:1. This calculator includes a preset for common fuel blends.

Can I use volume instead of mass?

No — AFR is always a mass ratio, not a volume ratio. Air density varies with temperature and altitude, so always use grams or kilograms (or any consistent mass unit) for both air and fuel inputs.

Air Fuel Ratio Calculator

Name: Air Fuel Ratio Calculator
Availability: InStock
Author: Nham Vu

Enter air mass and fuel mass (or use fuel type presets) to instantly calculate AFR, lambda, and mixture status.

Inputs

Fuel Type

Air Mass (any unit — g, kg, lb)

Fuel Mass (same unit as air mass)

Enter air mass and fuel mass, then click Calculate AFR.

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Summary

Enter air mass and fuel mass (or use fuel type presets) to instantly calculate AFR, lambda, and mixture status.

How it works

Select your fuel type to pre-fill the stoichiometric AFR target, or enter a custom value.
Enter the air mass flow and fuel mass flow (same units on both sides).
The calculator divides air mass by fuel mass to get the actual AFR.
Lambda is computed as actual AFR divided by the stoichiometric AFR for that fuel.
A color-coded result indicates whether the mixture is rich (lambda < 1), stoichiometric (lambda ≈ 1), or lean (lambda > 1).

Use cases

Tuning engine management systems (ECU) for optimal power or economy.
Diagnosing rich or lean conditions causing poor performance or high emissions.
Verifying wideband O2 sensor readings against calculated values.
Understanding fuel injection timing and duty-cycle adjustments.
Comparing gasoline, ethanol, diesel, and methanol fueling strategies.
Teaching combustion theory in automotive or mechanical engineering courses.

Frequently Asked Questions

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Last updated: 2026-05-29 · Reviewed by Nham Vu