What this calculator does
The stoichiometric air-fuel ratio is the mass of air needed to burn a given mass of fuel completely. Octane comes out at 15.03 to 1, and petrol is conventionally quoted as 14.7 because it is a blend rather than pure octane.
The ratio varies less across hydrocarbons than you might expect. Methane is 17.12 and octane 15.03, a spread of under 15% despite a sevenfold difference in molecular size. That is because the hydrogen-to-carbon ratio, which is what actually drives the air demand per kilogram, changes only slowly along the series.
The formula
The oxygen required per mole of fuel is the carbon count plus a quarter of the hydrogen count. Air is treated as one part oxygen to 3.76 parts nitrogen by mole, so the air mass follows from the molar masses of both. Dividing air mass by fuel mass gives the ratio.
| Term | Meaning |
|---|---|
| Stoichiometric AFR | The exact air needed, with no excess of either. Also called lambda equals 1. |
| Lambda (λ) | Actual AFR divided by stoichiometric. Above 1 is lean, below 1 is rich. |
| Rich mixture | Excess fuel. Produces more power briefly and more carbon monoxide. |
| Lean mixture | Excess air. Better economy but higher combustion temperatures and more NOx. |
The inputs explained
| Field | What to enter |
|---|---|
| Carbon atoms (α) | Carbon atoms in the fuel molecule. Octane is 8, methane 1. |
| Hydrogen atoms (β) | Hydrogen atoms. Octane is 18, methane 4. |
When to use it
Engine tuning
Fuel injection targets a ratio relative to stoichiometric, so the reference value has to be right for the fuel.
Switching fuels
Running an engine on LPG or methane needs a different air-fuel ratio, which is why conversions need recalibration.
Combustion system design
Burners and furnaces are sized around the air demand of their fuel.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does the ratio vary across hydrocarbons?
A range of carbon counts with hydrogen fixed at 18.
| Carbon atoms | Stoichiometric AFR (mass basis) | Oxygen required (a) | Fuel molar mass |
|---|---|---|---|
| C = 3 | 19.011 : 1 | 7.500 mol | 54.18 g/mol |
| C = 5 | 16.684 : 1 | 9.500 mol | 78.20 g/mol |
| C = 8 | 15.028 : 1 | 12.500 mol | 114.23 g/mol |
| C = 10 | 14.403 : 1 | 14.500 mol | 138.25 g/mol |
Questions
Why is petrol quoted as 14.7 rather than 15.0?
Because petrol is a blend of many hydrocarbons, not pure octane. The 14.7 figure is the accepted average for typical pump petrol. Pure octane calculates to 15.03, and ethanol-blended fuels sit lower still, around 14.1 for E10.
What does lambda mean?
The actual air-fuel ratio divided by the stoichiometric one. Lambda of 1 is exactly stoichiometric, above 1 is lean and below 1 is rich. Oxygen sensors report lambda, which is why the same sensor works regardless of which fuel the engine burns.
Why do engines run rich under load?
Extra fuel evaporates and cools the incoming charge and the exhaust valves, protecting components at high load. It also gives slightly more power. The cost is fuel economy and increased carbon monoxide, which is why it is used only when needed.
Why does the ratio differ between fuels?
Because it depends on the hydrogen-to-carbon ratio. Hydrogen needs about eight times its own mass in oxygen while carbon needs under three, so hydrogen-rich fuels demand more air per kilogram. Methane at 17.12 is the extreme case among common fuels.
For the balanced equation behind it, see the combustion reaction balancer. For gas volumes at standard conditions, see the STP conversion calculator.