What is the stoichiometric AFR for gasoline?

The theoretical stoichiometric AFR for gasoline (approximated as C8H18, isooctane) is 15.1:1, but real pump gasoline blends average 14.7:1 due to oxygenates and varying hydrocarbon chain lengths. 14.7 is the industry-standard value used by most ECUs and wideband controllers.

Why is ethanol's stoichiometric AFR so much lower than gasoline's?

Ethanol (C2H5OH) contains oxygen in its molecular structure, so it needs less atmospheric oxygen to combust completely. Its stoichiometric AFR is 9.0:1 — nearly 40% less air per unit of fuel. This means injectors and fuel pumps must flow significantly more fuel to achieve the same power output.

What lambda should I target for maximum power?

Wide-open-throttle power peaks are typically between λ 0.85–0.90 (AFR ≈ 12.5–13.2 for gasoline). Richer mixtures cool combustion but reduce efficiency; leaner mixtures risk detonation. The exact optimum depends on the engine, compression ratio, and fuel.

Can I use lambda values across different fuels?

Yes — that is the main advantage of lambda over AFR. A wideband sensor calibrated for gasoline reports a universal lambda value regardless of fuel. λ = 0.85 means 15% excess fuel on any fuel, making cross-fuel comparison straightforward.

What is the stoichiometric AFR for E85?

E85 (85% ethanol, 15% gasoline by volume) has a stoichiometric AFR of approximately 9.76:1. The exact value varies with ethanol content — real E85 ranges from E70 to E85 seasonally, so some ECUs allow the blend ratio to be specified.

Air-Fuel Stoichiometric Calculator

Name: Air-Fuel Stoichiometric Calculator
Availability: InStock
Author: Nham Vu

Select a fuel to see its stoichiometric AFR, then enter a measured AFR to calculate lambda and determine rich/lean mixture status.

Fuel & AFR Inputs

Fuel Type

Stoichiometric AFR 14.70 : 1

Measured (Actual) AFR

: 1

From a wideband O2 sensor or dyno sheet

Enter a measured AFR and press Calculate

Stoichiometric AFR Reference

Fuel	Stoich AFR	Formula	Notes
Gasoline (pump)	14.7 : 1	~C8H18 blend	Industry standard; varies ±0.2 by blend
Gasoline (isooctane)	15.1 : 1	C₈H₁₈	Pure isooctane reference fuel
Ethanol (E100)	9.0 : 1	C₂H₅OH	Internal O₂ reduces air needed
E85 blend	9.76 : 1	85% C₂H₅OH + 15% C₈H₁₈	Varies with actual ethanol content
Methanol (M100)	6.46 : 1	CH₃OH	Racing fuel; very rich injector demand
Diesel	14.5 : 1	~C12H26	Compression ignition; always runs lean at idle
CNG	17.2 : 1	CH₄ (methane)	Compressed natural gas vehicles
Hydrogen	34.3 : 1	H₂	Wide flammability range; very lean capable

Summary

Select a fuel to see its stoichiometric AFR, then enter a measured AFR to calculate lambda and determine rich/lean mixture status.

How it works

Select the fuel type from the dropdown — the stoichiometric AFR updates immediately.
The stoichiometric AFR shown is the chemically ideal air-to-fuel ratio by mass for that fuel.
Enter your measured (actual) AFR from a wideband O2 sensor or dyno data.
Lambda is computed as: λ = Actual AFR ÷ Stoichiometric AFR.
The result panel shows lambda, mixture status (rich/lean/stoich), and the percentage deviation from stoich.

Use cases

Verify wideband O2 sensor readings against known stoichiometric baselines.
Tune fuel maps for flex-fuel or alternative-fuel engine conversions.
Convert between lambda and AFR when switching fuel types on a standalone ECU.
Determine how rich or lean a target AFR is as a percentage for fueling calculations.
Classroom or self-study reference for combustion chemistry and engine management.

Frequently Asked Questions

Last updated: 2026-07-22 · Reviewed by Nham Vu