Air-Fuel Ratio (AFR

Stoichiometric AFR is the theoretically perfect ratio for complete combustion. Lambda (λ) = 1.0 at stoich. Rich engines (λ<1) produce more power but worse emissions. Lean (λ>1) improves fuel economy but can cause misfires.

To use the Air-Fuel Ratio (AFR) Calculator, enter your Air Mass Flow Rate, Fuel Mass Flow Rate, Fuel Type. The calculator instantly computes Air-Fuel Ratio (AFR), Lambda (λ) — Relative AFR and more. Results update in real time as you change any input — no submit button needed.

How to Use This Calculator

Enter your Air Mass Flow Rate (e.g. 14.7 in g/s (or any unit)), Fuel Mass Flow Rate (e.g. 1 in g/s (same unit)), and Fuel Type — choose from Gasoline / Petrol (Stoich = 14.7:1), Diesel (Stoich = 14.5:1), E85 Ethanol (Stoich = 9.8:1). Results update instantly as you type — adjust any value to compare scenarios.

The Formula

AFR = Air Mass ÷ Fuel Mass

Lambda (λ) = AFR ÷ Stoichiometric AFR

Stoich Gasoline = 14.7:1 | Diesel = 14.5:1 | Ethanol = 9.8:1

Worked Example

14.7g air / 1g gasoline: AFR = 14.7:

• Lambda = 14.7/14.7 = 1.

• Status: Stoichiometric — perfect for 3-way catalytic converter efficiency.

Understanding Your Result

Percentages are relative values — context determines what is "good" or "bad." A 50% discount is significant; a 50% failure rate is alarming. Percentage change and percentage points are different: moving from 4% to 5% is a 1 percentage point increase, but a 25% relative increase. When comparing percentages across different base values, always check whether absolute differences are meaningful — a 10% improvement on a large base creates more absolute value than 10% on a small base.