Air-Fuel Ratio Calculator

The air-fuel ratio (AFR) is the mass of air mixed with each part of fuel inside an engine's cylinder. That single number drives fuel economy, power output, and what comes out of the tailpipe. Enter any two of AFR, air mass, or fuel mass and the third value follows from AFR equals air mass divided by fuel mass.

What is air-fuel ratio?

AFR is a mass ratio. 14.7:1 means 14.7 grams of air for every gram of fuel. The stoichiometric AFR is the ratio where there's exactly enough oxygen to burn every molecule of fuel, with no leftover fuel and no leftover oxygen. The exact number depends on the fuel. Gasoline sits around 14.7:1, diesel near 14.6:1, ethanol at about 9:1 because its molecule already carries an oxygen atom, and pure hydrogen at 34.3:1 because it needs a lot of air per gram of fuel.

Run above stoichiometric and the mixture is lean. Run below it and the mixture is rich. Lean burns hotter and saves fuel but produces more NOx. Rich burns cooler and protects parts under heavy load, which is why factory engines briefly enrich the mixture at wide-open throttle.

How to use this calculator

First pick a fuel. That loads the stoichiometric AFR for reference, so you know what the engine actually wants. Then enter any two of AFR, mass of air, or mass of fuel. The remaining value computes from $\text{AFR} = \text{Mass of air} / \text{Mass of fuel}$ . Mass works in grams, kilograms, or pounds; the ratio is dimensionless so the answer stays consistent.

Where AFR shows up

Engine tuning. Modern ECUs target stoichiometric most of the time so the three-way catalyst can simultaneously reduce NOx and oxidize CO and hydrocarbons. Tuners deliberately bias rich or lean depending on the goal: race calibrations sit rich for thermal margin, economy maps sit lean for fuel savings.
Industrial burners. Boilers, kilns, and gas turbines run with a small percentage of excess air. Too much excess air carries heat out the flue; too little produces CO and soot. Combustion engineers usually settle on a few percent above stoichiometric.
Emissions diagnostics. A tailpipe AFR that drifts rich usually points to a leaking injector, failing fuel pressure regulator, or a lazy oxygen sensor. Lean drift more often traces back to a vacuum leak or a clogged injector.
Alternative fuels. E85 needs roughly 30 percent more fuel by mass than pump gasoline to reach stoichiometric, which is why flex-fuel vehicles use larger injectors and revised fuel maps.

Tips for tuning AFR

When cruising under light load on gasoline, a slightly lean mixture near 15.5:1 trims fuel use without hurting drivability.
At wide-open throttle, hold AFR between 12.5:1 and 13:1 to lower combustion temperatures and reduce the risk of detonation.
Diesel engines always run lean. AFR typically sits between 18:1 under heavy load and 60:1 or higher at idle, because a diesel controls power with fuel quantity rather than by throttling the incoming air.
For emissions compliance, hold lambda between 0.99 and 1.01. The three-way catalyst loses efficiency quickly outside that narrow window.

Frequently asked questions

What is lambda and how does it relate to AFR?

Lambda (λ) is the actual AFR divided by the stoichiometric AFR for the same fuel. Lambda equal to 1.00 is stoichiometric, less than 1.00 is rich, greater than 1.00 is lean. For gasoline, an actual AFR of 13.2:1 works out to lambda = 13.2 / 14.7 = 0.90, or about ten percent rich. Lambda has the advantage of being fuel-agnostic, which is why most wideband sensors display lambda directly.

Why do different fuels have different stoichiometric ratios?

Two things drive it: the carbon-to-hydrogen ratio inside the molecule and whether the molecule already contains oxygen. Methane is mostly hydrogen, so it needs more air per gram than longer hydrocarbons. Ethanol carries an oxygen atom inside its molecule, so less external air is needed for complete combustion. Pure hydrogen has the highest stoichiometric AFR of any common fuel because it has no carbon and the lightest possible fuel atom.

Can I use this calculator for rocket engines?

Not directly. Rockets carry their own oxidizer instead of breathing air, so the relevant number is the oxidizer-to-fuel ratio (O/F) rather than AFR. The math is the same kind of mass ratio, but the reference numbers change. Kerosene with liquid oxygen runs around O/F 2.3; hydrogen with liquid oxygen runs around 5 to 6.