What Is an Air-Fuel Ratio Calculator?
An Air-Fuel Ratio Calculator determines the ratio between the mass of air and the mass of fuel in a mixture. The result is commonly written as a ratio such as 14.7:1, meaning 14.7 units of air mass for every 1 unit of fuel mass.
This calculator also compares the calculated AFR with a selected stoichiometric reference and calculates lambda, making it easier to express the mixture relative to that reference.
Air-Fuel Ratio Formula
AFR is calculated by dividing air mass by fuel mass. Because AFR is a mass ratio, both values need to use the same mass unit.
AFR = Mass of Air ÷ Mass of Fuel
For example, if the mixture contains 147 grams of air and 10 grams of fuel, dividing 147 by 10 produces an AFR of 14.7:1.
How to Use the Air-Fuel Ratio Calculator
Enter Air Mass
Enter the amount of air by mass using grams, kilograms or another consistent mass unit.
Enter Fuel Mass
Enter fuel mass using exactly the same mass unit used for the air value.
Calculate AFR
Select a reference fuel and calculate AFR, lambda and the relative mixture.
Air-Fuel Ratio Calculation Example
Suppose the measured mixture contains 147 grams of air and 10 grams of gasoline by mass.
Example Calculation
Using a 14.7:1 gasoline reference, this example produces lambda 1.00.
What Is Lambda in Air-Fuel Ratio?
Lambda expresses the actual AFR relative to a selected stoichiometric AFR. A lambda value of 1.00 means the actual AFR matches the reference stoichiometric ratio.
λ = Actual AFR ÷ Stoichiometric AFR
Lambda = 1
The calculated AFR equals the selected stoichiometric reference.
Lambda Below 1
The mixture contains more fuel relative to air than the selected stoichiometric reference, often described as richer.
Lambda Above 1
The mixture contains less fuel relative to air than the selected stoichiometric reference, often described as leaner.
AFR vs Lambda
AFR is fuel-dependent, while lambda expresses the mixture relative to that fuel's selected stoichiometric reference.
Common Stoichiometric AFR Reference Values
Different fuels have different stoichiometric air-fuel ratios. The following values are useful approximate references rather than universal specifications for every fuel blend.
| Fuel | Approx. Stoichiometric AFR |
|---|---|
| Gasoline | 14.7:1 |
| Diesel | Approx. 14.5:1 |
| E85 | Approx. 9.77:1 |
| Ethanol | Approx. 9.0:1 |
| Methanol | Approx. 6.47:1 |
Real commercial fuels can vary in composition. For example, the actual ethanol content of a fuel blend may differ, which changes its exact stoichiometric AFR. Use the custom option when you have a specific reference value.
Rich vs Lean Air-Fuel Ratio
Rich and lean are relative terms. A mixture is considered richer than the selected stoichiometric reference when its lambda is below 1. A mixture is leaner when lambda is above 1.
For a 14.7:1 gasoline reference, an AFR of 13:1 is richer than stoichiometric, while an AFR of 16:1 is leaner. Those labels alone do not determine whether a mixture is appropriate for a particular engine or operating condition.
Why Air-Fuel Ratio Matters
Air-fuel ratio is one way to describe the relationship between air and fuel supplied to a combustion process. In engine applications, AFR can vary with operating conditions, fuel type, engine design and control strategy.
Fuel Type
Different fuels require different reference ratios, so the same numerical AFR does not represent the same lambda for every fuel.
Lambda Comparison
Lambda provides a convenient way to compare a mixture against its selected stoichiometric reference.
This tool calculates mathematical AFR and lambda values only. It does not recommend an engine tuning target or determine a safe operating mixture. Appropriate engine settings depend on the engine, fuel, load, calibration, emissions system and manufacturer requirements.
AFR by Mass vs Volume
Air-fuel ratio is normally expressed as a mass ratio, not a direct volume ratio. Therefore, entering 10 liters of one material and 1 liter of another does not automatically represent a 10:1 AFR.
For this calculator, enter both quantities using the same mass unit. You can use grams, kilograms, pounds or another mass unit because the identical units cancel when the ratio is calculated.
Common Air-Fuel Ratio Calculation Mistakes
- Using air mass and fuel mass in different units.
- Confusing a mass ratio with a volume ratio.
- Using gasoline's stoichiometric AFR for a different fuel.
- Treating AFR and lambda as identical numbers.
- Assuming a mathematical rich or lean label automatically determines an appropriate engine setting.
Air-Fuel Ratio Calculator FAQs
Divide the mass of air by the mass of fuel. For example, 147 grams of air divided by 10 grams of fuel gives an AFR of 14.7:1.
It means there are 14.7 units of air mass for every one unit of fuel mass. It is commonly used as an approximate stoichiometric reference for gasoline.
Divide the actual AFR by the selected stoichiometric AFR. For example, 14.7 divided by 14.7 gives lambda 1.00.
Lambda 1 means the calculated air-fuel ratio equals the selected stoichiometric AFR reference.
No. Different fuels and blends have different stoichiometric air-fuel ratios. The exact value also depends on fuel composition.
Yes. You can use grams, kilograms, pounds or another mass unit as long as both the air mass and fuel mass are entered using the same unit.