Compute the air-to-fuel ratio, lambda, and equivalence ratio from the mass of air and fuel, with a grouped database of 22 fuels and target-lambda tuning guidance.
| Quantity | Value | Formula | Notes |
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| Mixture | AFR range (: 1) | Lambda range | Typical use |
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| Metric | Value | Notes |
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Founder & CEO, Toolraxy
Faiq Ur Rahman is a web designer, digital product developer, and founder of Toolraxy, a growing platform of web-based calculators and utility tools. He specializes in building structured, user-friendly tools focused on health, finance, productivity, and everyday problem-solving.
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Air-fuel ratio (AFR) is the mass of air divided by the mass of fuel entering a combustion event. At stoichiometric AFR, the oxygen in the air exactly consumes the fuel, leaving neither excess fuel nor excess oxygen in an ideal reaction. For gasoline, that ratio is approximately 14.7 kilograms of air per kilogram of fuel .
AFR is a mass ratio, not a volume ratio. That distinction matters because air and fuel densities differ by orders of magnitude. A 14.7:1 gasoline AFR means 14.7 grams of air for every 1 gram of fuel, regardless of how much space each occupies before combustion. The convention is universal in engine calibration and emissions testing, and lambda is simply AFR normalised by the fuel’s stoichiometric value: λ = AFR ÷ AFR_stoich .
AFR is air mass divided by fuel mass. To use the calculator, enter the fuel you burned, confirm or override its stoichiometric AFR, input the measured mass of fuel and mass of air, and set the lambda you were targeting. The tool returns the actual AFR and the fuel mass you would have needed to hit that target.
Select a fuel from the grouped list, or choose Custom fuel and type your own stoichiometric AFR.
Enter the mass of fuel consumed, in kilograms.
Enter the mass of air consumed, in kilograms.
Set the target lambda you were aiming for (1.00 for stoichiometric, 0.85 for rich power, 1.10 for lean cruise).
Read the hero AFR value and the lambda tile. If the verdict banner says “Rich” or “Lean,” the numbers are doing their job.
Check the “Fuel for target λ” tile. That is the fuel mass that would have hit your target with the air you actually moved.
Compare it with the “Δ fuel vs target” row in the breakdown table to see how many kilograms rich or lean you ran.
The core AFR formula is a division: AFR = mass of air ÷ mass of fuel. Lambda adds one more step: λ = actual AFR ÷ stoichiometric AFR. Equivalence ratio is the reciprocal: φ = 1 ÷ λ. The calculator uses these three relationships directly, with no correction factors for fuel density, humidity, or residual gas.
Formula: AFR = m_air ÷ m_fuel
Formula: λ = AFR ÷ AFR_stoich
Formula: φ = 1 ÷ λ
Formula: m_fuel,target = m_air ÷ (λ_target × AFR_stoich)
The target-fuel formula rearranges the lambda definition to solve for fuel mass when you fix air mass and target lambda. If you moved 15.09 kg of air and wanted λ = 1.00 on gasoline (stoich 14.7), the required fuel is 15.09 ÷ (1.00 × 14.7) = 1.027 kg. If you actually burned 1.000 kg, the delta is −0.027 kg: you were lean by 27 grams of fuel.
A datalog from a 2.0 L turbo gasoline engine shows 1.000 kg of fuel consumed over a pull and 12.100 kg of air metered by the MAF sensor. The fuel is pump gasoline with a stoichiometric AFR of 14.7. The tuner was targeting λ = 0.85 for maximum power.
Step 1 – Actual AFR: 12.100 ÷ 1.000 = 12.10:1.
Step 2 – Lambda: 12.10 ÷ 14.7 = 0.823.
Step 3 – Equivalence ratio: 1 ÷ 0.823 = 1.215.
Step 4 – Fuel for target λ: 12.100 ÷ (0.85 × 14.7) = 12.100 ÷ 12.495 = 0.968 kg.
Step 5 – Delta: 1.000 − 0.968 = +0.032 kg. The engine ran 32 grams of fuel rich against the 0.85 target.
Interpretation: λ = 0.823 is richer than the 0.85 target and sits inside the “Rich” band. The verdict banner says “Rich: the mixture is rich of stoichiometric.” A tuner would pull roughly 3% fuel from the table at that load cell.
AFR shows up wherever a combustion engine is calibrated, tested, or taught. The settings range from a dyno cell with a wideband in the downpipe to a classroom where the stoichiometry is on a whiteboard. Each setting reads the same three numbers differently.
Calibrators use lambda as the primary target because it survives fuel changes. A gasoline tune at λ = 0.85 and an E85 tune at λ = 0.85 are both rich by the same percentage, even though the AFR numbers are 12.5 and 8.3 respectively . The AFR calculator converts the lambda target into a fuel mass, which is what the ECU fuel table actually stores. A dyno operator logging 1.2 kg of fuel against 11.8 kg of air can check in seconds whether the pull was inside the safe rich band.
Three-way catalysts convert CO, hydrocarbons, and NOx most efficiently in a narrow window around λ = 1.00 . An emissions bench that reports AFR in gasoline-equivalent numbers still needs the actual lambda to know whether the catalyst is being poisoned by a rich excursion or starved by a lean one. The calculator’s equivalence ratio tile gives the same information in the phi convention used by some combustion codes.
A flex-fuel vehicle’s stoichiometric AFR moves with ethanol content. E85 is approximately 9.8:1, E10 is approximately 14.1:1, and gasoline is 14.7:1 . When a tuner switches fuels without updating the stoich input, the lambda calculation drifts. The calculator’s fuel list makes the correct stoich value a dropdown selection instead of a memory exercise. For a blend that isn’t on the list, the Custom fuel option accepts any positive stoichiometric value.
Motorcycle, kart, and powersport engines often run without a factory MAF sensor. A builder who knows fuel mass from a flow test and air mass from a bench-flow estimate can still compute AFR and lambda before the first track session. The rich-power band around λ = 0.85 is a starting point, not a guarantee, but it is a defensible baseline for a new build.
The table below lists the stoichiometric AFR values the calculator uses for its 22-fuel database. Values are typical mass ratios from combustion reference tables; actual pump fuel can vary slightly with additive packages and blend stocks .
| Fuel | Stoichiometric AFR (kg air / kg fuel) |
|---|---|
| Gasoline (petrol) | 14.7 |
| E10 (10% ethanol) | 14.1 |
| E15 (15% ethanol) | 13.9 |
| E25 (25% ethanol) | 13.3 |
| E50 (50% ethanol) | 11.5 |
| E85 (85% ethanol) | 9.8 |
| AVGAS 100LL | 14.7 |
| Ethanol (E100) | 9.0 |
| Methanol (M100) | 6.4 |
| Jet A / Kerosene | 15.0 |
| Diesel | 14.5 |
| Biodiesel (B100) | 12.5 |
| Propane (LPG) | 15.7 |
| Butane | 15.4 |
| Natural gas (CNG / methane) | 17.2 |
| LNG (liquefied natural gas) | 17.2 |
| Hydrogen (H₂) | 34.3 |
| Nitromethane | 1.7 |
| Acetylene | 13.2 |
| Ammonia (NH₃) | 6.06 |
| Wood gas | 5.7 |
| Coal gas | 3.5 |
For gasoline, 14.7:1 is stoichiometric, but “good” depends on the goal. Maximum power usually sits around 12.5:1 (λ ≈ 0.85), and best economy sits around 15.4–16.2:1 (λ ≈ 1.05–1.10) . There is no single good number without a target.
Gasoline’s stoichiometric AFR is approximately 14.7 kilograms of air per kilogram of fuel . Pump gasoline varies slightly with ethanol content and additives, which is why E10 sits near 14.1 and E85 near 9.8.
AFR = λ × stoichiometric AFR. If lambda is 0.85 and gasoline’s stoich is 14.7, AFR = 0.85 × 14.7 = 12.495:1. The calculator does this division in reverse: it takes the measured AFR and divides by stoich to return lambda.
Lambda is actual AFR divided by stoichiometric AFR. Equivalence ratio is the reciprocal: stoichiometric AFR divided by actual AFR, or 1 ÷ λ. Lambda 0.85 and phi 1.18 describe the same rich mixture. The calculator shows both.
E85 is 85% ethanol and 15% gasoline by volume, and ethanol carries oxygen in its molecule. That built-in oxygen means less air mass is needed to burn a kilogram of E85, so its stoichiometric AFR is about 9.8:1 instead of 14.7:1 .
Yes, if you enter diesel’s stoichiometric AFR of approximately 14.5:1 and a diesel-appropriate target lambda. Diesel engines normally run lean of stoichiometric at part load, often λ = 1.5–3.0 or higher, and the calculator’s mixture scale includes an “Ultra lean” band for that range.
A rich mixture has more fuel than stoichiometric, so lambda is below 1.00 and AFR is below the stoichiometric value. Rich mixtures cool the combustion chamber and can suppress knock, but they raise CO and hydrocarbon emissions .
A lean mixture has more air than stoichiometric, so lambda is above 1.00 and AFR is above the stoichiometric value. Lean mixtures improve fuel economy up to a point, but leaner than roughly λ = 1.20 can cause misfire and high NOx in gasoline engines .
The arithmetic is exact to the displayed decimals. The accuracy of the result depends entirely on the accuracy of the fuel mass, air mass, and stoichiometric value you enter. A calculator cannot correct a drifting MAF sensor or a miscalibrated fuel flow meter.
It works for any blend if you know the blend’s stoichiometric AFR and enter it as a Custom fuel. The preset E10, E15, E25, E50, and E85 options cover common ethanol blends. It does not read a flex-fuel sensor or adjust stoich automatically.
It is the fuel mass that would have hit your target lambda given the air mass you entered. If the actual fuel mass is higher, you ran rich; if lower, you ran lean. The difference is shown as “Δ fuel vs target” in kilograms.
Yes. Enter the gauge’s AFR reading as mass of air and 1.000 kg as mass of fuel, or better, use the lambda reading directly as the target and compare the calculator’s lambda output. Remember that a wideband configured to gasoline stoich will display 14.7 at stoichiometry even on E85 .
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