What is the air-fuel ratio (AFR)?

AFR is the ratio of air mass to fuel mass in a combustion mixture. It is dimensionless and expressed as a ratio such as 14.7:1, meaning 14.7 g of air per 1 g of fuel. A correct AFR is critical for complete combustion, engine efficiency, emissions and catalyst function.

What is stoichiometric AFR and why does it matter?

The stoichiometric AFR is the theoretically perfect ratio at which all fuel and all oxygen are consumed simultaneously, leaving no excess of either. For gasoline it is 14.7:1. At this point a three-way catalytic converter operates with maximum efficiency, reducing CO, HC and NOx simultaneously. Deviating from stoichiometry shifts the engine toward power (rich) or economy (lean).

What is lambda (λ) and how is it different from AFR?

Lambda is the normalised air-fuel ratio — it is the actual AFR divided by the stoichiometric AFR for the fuel in use. Lambda = 1.0 means stoichiometric regardless of fuel type. Lambda below 1 is rich; above 1 is lean. Using lambda allows direct comparison across fuels (petrol, diesel, E85, etc.) without needing to remember each fuel's stoichiometric value.

What is the equivalence ratio (φ)?

The equivalence ratio φ (phi) is simply 1 / λ. It is preferred in combustion research because φ greater than 1 means rich and φ less than 1 means lean — which some engineers find more intuitive. The calculator shows both so you can use whichever convention your ECU or dyno software uses.

Why does stoichiometric AFR differ between fuels?

Each fuel has a different hydrogen-to-carbon ratio and oxygen content, so the mass of air needed to combust one unit of fuel differs. Gasoline (CₙH₁.₈₇ₙ) needs 14.7 parts air per part fuel; pure ethanol (C₂H₅OH) already contains oxygen so only 9.0 parts air are needed; hydrogen (H₂) has no carbon at all and needs 34.3 parts air. Blends like E85 sit between their constituents at about 9.76:1.

What AFR should I target for maximum power versus fuel economy?

For maximum power most petrol engines target a slightly rich mixture of λ 0.85–0.90 (AFR roughly 12.5–13.2:1 for gasoline) to keep combustion temperatures down and avoid detonation. For best fuel economy at light-to-moderate load, modern engines run slightly lean at λ 1.05–1.15. Stoichiometric (λ 1.0) is the mandatory target whenever the catalytic converter is active.

Air-Fuel Ratio Calculator

The air-fuel ratio (AFR) is the single most important combustion parameter in any internal-combustion engine, boiler or furnace. It determines whether a mixture burns completely, how much power it produces, how cleanly it burns, and whether an emissions control system (catalytic converter, DPF, SCR) can function correctly. This calculator handles every fuel type commonly encountered in automotive tuning, motorsport, alternative fuels research and industrial combustion engineering.

How it works

The fundamental formula is simple:

AFR = mass of air / mass of fuel

From this, two normalised quantities are derived:

Lambda (λ) = AFR / stoichiometric AFR for the fuel in use. Lambda of exactly 1.0 means stoichiometric — a perfect chemically balanced mixture — regardless of which fuel you are burning. Lambda below 1 is rich (excess fuel); above 1 is lean (excess air).
Equivalence ratio (φ) = 1 / λ. Preferred in combustion research and in some ECU platforms. Values above 1 are rich; values below 1 are lean — the inverse of lambda.

The calculator lets you solve in four directions: compute AFR from two measured masses; find the air mass needed for a target AFR; find the fuel mass for a target AFR; or convert a wideband lambda sensor reading directly into AFR and φ.

Stoichiometric AFR by fuel

Each fuel’s stoichiometric AFR follows from its molecular formula and the oxygen-balance equation. Gasoline is modelled as a C₁H₁.₈₇ hydrocarbon chain giving 14.7:1. Ethanol (C₂H₅OH) already carries an oxygen atom, so it needs less external air — 9.0:1 for pure ethanol. Hydrogen (H₂) burns to water with no carbon, requiring 34.3:1 air by mass. Blended fuels like E85 (85 % ethanol, 15 % gasoline by volume) sit at approximately 9.76:1 depending on exact blend composition and ethanol density.

Worked example

A tuner measures 197 g of air entering a cylinder and 13.4 g of fuel injected per cycle on a turbocharged petrol engine:

AFR = 197 / 13.4 = 14.70:1 (right at stoichiometric for gasoline)
Lambda = 14.70 / 14.7 = 1.000
Equivalence ratio = 1 / 1.000 = 1.000

Now suppose the tuner wants a power-enrichment map at λ = 0.88 (slightly rich, for a boosted run):

Target AFR = 0.88 × 14.7 = 12.94:1
With 197 g of air, required fuel = 197 / 12.94 = 15.22 g per cycle
That is 13.6 % more fuel than stoichiometric

The colour-coded mixture indicator and lambda scale let you immediately see where your calibration sits relative to the catalyst-operation window, the peak-power zone and the misfire-risk boundary.

Formula note

All calculations use:

λ = AFR⊂actual / AFR⊂stoich
φ = 1 / λ
air mass = AFR × fuel mass
fuel mass = air mass / AFR

No empirical correlations or approximations are involved — these are exact definitions from SAE J1297 and ISO 8178 combustion standards. Stoichiometric values sourced from Heywood, Internal Combustion Engine Fundamentals (McGraw-Hill, 1988) and updated with NIST thermochemical data for alternative fuels.