Calculate your actual air-fuel ratio, compare it with stoichiometric AFR for the selected fuel, and see if the mix is rich, near stoichiometric, or lean.
Advanced options
How to use our Air Fuel Ratio Calculator
- Choose Fuel type, then enter Air amount (lb) and Fuel amount (lb) using the same mass basis for both values.
- Leave Input basis on mass ratio, because this calculator compares air and fuel by mass, not by volume.
- If you picked Fuel type = Custom stoich value, enter Custom stoichiometric AFR for your blend or tuner-supplied reference.
- Click Calculate to see Actual air-fuel ratio, Mix status, Lambda, and the signed Difference from stoich.
- Sanity-check the result: if Actual air-fuel ratio matches Stoichiometric AFR used, lambda should be about 1, Difference from stoich should be about 0%, and the mix should read near stoichiometric.
Definitions
Air-fuel ratio (AFR): The mass of air divided by the mass of fuel. In this calculator, Actual air-fuel ratio = Air amount (lb) divided by Fuel amount (lb).
Stoichiometric AFR: The exact air-fuel ratio for complete burn of a specific fuel. Rich and lean are judged against this reference, and the reference changes with fuel type [3].
Rich: The actual AFR is lower than stoichiometric. That means there is less air relative to fuel than the stoich target [2].
Lean: The actual AFR is higher than stoichiometric. That means there is more air relative to fuel than the stoich target [1].
Lambda: A fuel-normalized comparison equal to actual AFR divided by stoichiometric AFR. Lambda = 1 is stoichiometric, below 1 is rich, and above 1 is lean.
Equivalence ratio: The inverse of lambda. A value above 1 means rich, and below 1 means lean.
Difference from stoich: The signed percent difference between your actual AFR and the stoichiometric AFR used. Negative means richer than stoich, positive means leaner.
Excess air: The percent amount of air above stoichiometric. For rich mixtures, this becomes negative and represents an air deficit relative to stoich.
Common mistakes and quick fixes
Mistake: Entering volume instead of mass in Air amount (lb) or Fuel amount (lb) .
Fix: Use pounds of air and pounds of fuel, or convert your measurements to mass before calculating.
Mistake: Using different unit types for Air amount (lb) and Fuel amount (lb) .
Fix: Keep both inputs on the same mass basis. If air is in lb, fuel must also be in lb.
Mistake: Choosing the wrong Fuel type and then judging the mix from the wrong reference.
Fix: Pick the fuel that matches what is actually in the engine, because Stoichiometric AFR used changes by fuel.
Mistake: Leaving Custom stoichiometric AFR blank after selecting Custom stoich value.
Fix: Enter a stoich AFR greater than 0, or switch Fuel type back to a preset fuel.
Mistake: Reading a negative Difference from stoich or negative Excess air as an error.
Fix: Negative values are valid here. They mean the mixture is richer than stoichiometric and has an air deficit relative to stoich.
Mistake: Treating Lambda as the same number as Actual air-fuel ratio .
Fix: Use Actual air-fuel ratio for the raw ratio and Lambda for the fuel-normalized comparison. Lambda near 1 means near stoichiometric for that fuel.
Limitations & Key Assumptions / Boundary Conditions
- This is a mass-basis calculator. It assumes Air amount (lb) and Fuel amount (lb) are both true masses and use the same unit basis.
- Fuel presets use fixed stoichiometric AFR reference values. Real fuels, especially blended fuels like E85, can vary enough that a custom stoich value may be more accurate.
- Near-stoichiometric labeling depends on the calculator's comparison window, so values very close to stoich may be grouped as near stoichiometric instead of strictly rich or lean.
- The tool compares ratios only. It does not model temperature, pressure, sensor error, injector behavior, incomplete combustion, or engine tuning effects.
- Negative Excess air is mathematically valid for rich mixtures, but many people use excess-air language mainly for lean cases.
- If your air or fuel values came from volume, flow, or sensor readings converted with assumptions, the final AFR result is only as accurate as those conversions.
Methodology
How the calculator works
The tool first computes the actual air-fuel ratio from the two masses you enter. It then looks up the stoichiometric AFR for the selected fuel, or uses your custom stoich value when you choose that option. From those two ratios, it calculates lambda, equivalence ratio, signed percent difference, and stoichiometric air or fuel needed for comparison.
Formulas used
AFR_actual = m_air / m_fuel
λ = AFR_actual / AFR_stoich
φ = AFR_stoich / AFR_actual = 1 / λ
Difference from stoich (%) = ((AFR_actual - AFR_stoich) / AFR_stoich) x 100
Excess air (%) = (λ - 1) x 100
Air needed at stoich = AFR_stoich x m_fuel
Fuel needed at stoich = m_air / AFR_stoich
Preset stoichiometric AFR values
The calculator uses these mass-basis reference values: gasoline 14.7, diesel 14.5, E10 14.08, E85 9.76, ethanol 9.0, methanol 6.47, and hydrogen 34.3. These are common reference values used to compare measured AFR across fuels.
How mix status is decided
If the actual AFR is below the stoichiometric AFR, the mix is rich. If it is above, the mix is lean. If it is extremely close to the stoich target, the calculator can label it near stoichiometric so tiny rounding differences do not flip the result back and forth [1][3].
Worked mini-example
Example: choose gasoline, enter Air amount (lb) = 12.5 and Fuel amount (lb) = 1. Gasoline stoich AFR is 14.7.
AFR_actual = 12.5 / 1 = 12.5
λ = 12.5 / 14.7 = 0.8503
φ = 14.7 / 12.5 = 1.176
Difference from stoich = ((12.5 - 14.7) / 14.7) x 100 = -14.97%
Excess air = (0.8503 - 1) x 100 = -14.97%
Interpretation: the actual AFR is below stoich, lambda is below 1, and the signed percent is negative, so the mixture is rich.
Assumptions behind the math
This calculator assumes positive numeric mass inputs, a valid stoichiometric AFR greater than zero, and no hidden unit conversion. It does not estimate AFR from exhaust sensors, chemistry formulas, or volumetric flow. If your fuel is a custom blend, the most accurate comparison comes from entering a known custom stoichiometric AFR instead of relying on the nearest preset.