AFR Calculator - Air-Fuel Ratio

Calculate air-fuel ratio, lambda, and stoichiometric AFR for gasoline, diesel, ethanol, CNG, hydrogen, and custom fuels.

⛽ Air-Fuel Ratio Calculator
Air Mass
g
Fuel Mass
g
Fuel Type (lambda reference)
Actual Air-Fuel Ratio
: 1
Fuel Type
Fuel Type or Custom Formula
Custom Fuel Formula (CHO only)
Air-Fuel Ratio
—
Lambda (λ)
—
Mixture
—
Stoich AFR
—
Fuel Molar Mass
—
O₂ per kg Fuel
—

⛽ What is the Air-Fuel Ratio (AFR)?

The air-fuel ratio (AFR) is the mass ratio of air to fuel supplied to a combustion engine or burner. An AFR of 14.7 : 1 means that 14.7 kg of air are mixed with every 1 kg of gasoline. This ratio is the single most important parameter controlling combustion quality, engine power, fuel efficiency, and exhaust emissions.

Every fuel has a stoichiometric AFR — the theoretical ratio at which all the fuel and all the oxygen are consumed exactly, leaving no excess of either. For gasoline the stoichiometric AFR is 14.7, for diesel it is 14.5, for ethanol it is 9.0, and for hydrogen it is 34.3. Running above the stoichiometric ratio produces a lean mixture; running below produces a rich mixture.

In engine calibration, engineers use lambda (λ) rather than AFR directly, because lambda normalises the ratio against the stoichiometric value: λ = AFR / AFR_stoich. A lambda of 1.0 means perfect stoichiometry regardless of fuel type. Rich mixtures have λ less than 1.0; lean mixtures have λ greater than 1.0. This makes lambda a fuel-independent measure of mixture quality.

This calculator covers three practical use cases. The AFR from Masses mode computes AFR and lambda from measured air and fuel flow rates. The Lambda mode converts a known AFR into lambda using any fuel's stoichiometric reference. The Stoich AFR mode calculates the theoretical stoichiometric AFR from first-principles combustion chemistry for any CHO compound, using the combustion equation CxHyOz plus (x + y/4 minus z/2) moles of O2 to produce CO2 and H2O.

📐 Formulas

AFR = mair ÷ mfuel
mair = mass of air (any unit — g, kg, g/s, kg/h)
mfuel = mass of fuel (same unit as air)
Example: 147 g air and 10 g fuel → AFR = 147 / 10 = 14.7 : 1
λ = AFRactual ÷ AFRstoich
AFRactual = measured or calculated air-fuel ratio
AFRstoich = stoichiometric AFR for the fuel (14.7 for gasoline)
λ < 1 = rich mixture (excess fuel)
λ > 1 = lean mixture (excess air)
λ = 1 = stoichiometric
AFRstoich = [(x + y/4 − z/2) × 32 ÷ 0.232] ÷ Mfuel
x, y, z = carbon, hydrogen, oxygen atom counts in the fuel formula CxHyOz
32 = molar mass of O2 (g/mol)
0.232 = mass fraction of O2 in dry air (23.2 %)
Mfuel = molar mass of fuel (g/mol)
Example CH4: x=1, y=4, z=0 → (2 × 32 / 0.232) / 16.043 = 17.19 : 1

📖 How to Use This Calculator

Steps

1
Select the calculation mode — Choose AFR from Masses if you have measured air and fuel flow, Lambda to convert a known AFR into the excess air ratio, or Stoich AFR to compute the theoretical air requirement for a fuel.
2
Enter the input values — For AFR mode, type the air mass and fuel mass in any consistent unit such as grams or kilograms. For Lambda mode, enter your actual AFR value. For Stoich mode, pick a preset fuel or type a custom CHO formula such as C8H18 or C2H6O.
3
Select the fuel type — Choose gasoline, diesel, ethanol, methanol, LPG, CNG, or hydrogen to set the stoichiometric AFR reference for lambda calculation. In Stoich mode, select Custom Formula to enter any CxHyOz compound.
4
Read the results — The calculator shows AFR, lambda, and the mixture classification as rich, lean, or stoichiometric. In Stoich mode it also shows fuel molar mass and O2 per kg of fuel consumed.

💡 Example Calculations

Example 1 — Stoichiometric Gasoline Engine (AFR Mode)

147 g air and 10 g fuel for a gasoline engine

1
Air mass = 147 g, Fuel mass = 10 g, Fuel type = Gasoline.
2
AFR = 147 / 10 = 14.70 : 1.
3
Stoichiometric AFR for gasoline = 14.70 : 1.
4
Lambda = 14.70 / 14.70 = 1.000 → Stoichiometric (λ ≈ 1).
AFR = 14.70 : 1 | Lambda = 1.000 | Mixture: Stoichiometric
Try this example →

Example 2 — Rich Ethanol Mixture (Lambda Mode)

Actual AFR of 7.2 : 1 with ethanol fuel

1
Actual AFR = 7.2 : 1, Fuel type = Ethanol E100.
2
Stoichiometric AFR for ethanol = 9.00 : 1.
3
Lambda = 7.2 / 9.0 = 0.800.
4
Lambda = 0.800, which is less than 0.95, so the mixture is Rich (λ < 1). The engine is running with 20% excess fuel.
Lambda = 0.800 | Mixture: Rich (λ < 1)
Try this example →

Example 3 — Stoichiometric AFR for Methane from Formula (Stoich Mode)

Custom formula CH4 (natural gas / CNG)

1
Methane CH4: x = 1 carbon, y = 4 hydrogen, z = 0 oxygen.
2
O2 moles needed = x + y/4 - z/2 = 1 + 4/4 - 0 = 2.000 mol O2 per mol fuel.
3
O2 mass = 2.000 × 31.998 = 63.996 g/mol. Air mass = 63.996 / 0.232 = 275.84 g/mol.
4
Fuel molar mass = 1 × 12.011 + 4 × 1.008 = 16.043 g/mol.
5
Stoich AFR = 275.84 / 16.043 = 17.19 : 1. O2 per kg fuel = 63.996 / 16.043 = 3.989 kg/kg.
Stoich AFR = 17.19 : 1 | O2/kg fuel = 3.989 kg/kg | Molar mass = 16.043 g/mol
Try this example →

❓ Frequently Asked Questions

What is the air-fuel ratio (AFR)?+
AFR is the mass ratio of air to fuel entering the combustion chamber. An AFR of 14.7 : 1 means 14.7 kg of air for every 1 kg of gasoline. AFR determines combustion completeness, power output, fuel consumption, and exhaust emissions. It is measured by a wideband oxygen sensor and controlled by the engine control unit (ECU).
What is the stoichiometric AFR for gasoline?+
The stoichiometric AFR for gasoline is 14.7 : 1. This represents the theoretically perfect ratio where all fuel carbon and hydrogen combine with all available oxygen, producing CO2 and H2O with no excess of either reactant. Real gasoline is a blend of hydrocarbons, so 14.7 is an empirical standard rather than a calculated value for a single compound.
What does lambda (λ) mean in engine tuning?+
Lambda is the dimensionless excess air coefficient: λ = AFR / AFR_stoich. At lambda 1.0 the mixture is stoichiometric. Below 1.0 is rich (excess fuel); above 1.0 is lean (excess air). Lambda allows comparison across fuel types — a gasoline engine and an ethanol engine both running at lambda 0.85 are equally rich relative to their respective fuels, even though their actual AFR values differ.
What is a rich mixture and when is it used?+
A rich mixture (lambda less than 1.0, AFR below stoichiometric) has excess fuel. Rich combustion produces more power up to about lambda 0.85 to 0.90, lowers peak combustion temperature (protecting pistons and valves), and causes higher CO and unburned HC in exhaust. Turbocharged performance engines often run rich at full load to control exhaust gas temperatures below 950 degrees Celsius.
What is a lean mixture and when is it used?+
A lean mixture (lambda greater than 1.0, AFR above stoichiometric) has excess air. Lean combustion improves fuel economy, reduces CO emissions, and is used by modern GDI engines at light load — typically lambda 1.3 to 2.0. Very lean mixtures risk misfires and elevated NOx due to higher combustion temperatures at moderate lean ratios (around lambda 1.1 to 1.2).
Why does ethanol have a lower stoichiometric AFR than gasoline?+
Ethanol (C2H6O) contains one oxygen atom per molecule, which provides part of the oxidiser internally. This means less atmospheric oxygen (and therefore less air) is needed to complete combustion. The stoichiometric AFR for pure ethanol is 9.0 : 1, compared to 14.7 for gasoline. Flex-fuel vehicles running on E85 need injectors calibrated to deliver roughly 35% more fuel mass to maintain lambda 1.0.
How do I calculate stoichiometric AFR from a chemical formula?+
For any fuel formula CxHyOz, the combustion equation is: CxHyOz + (x + y/4 - z/2) O2 → x CO2 + (y/2) H2O. The stoichiometric AFR equals the required air mass divided by the fuel molar mass: AFR = [(x + y/4 - z/2) × 31.998 / 0.232] / (12.011x + 1.008y + 15.999z). The Stoich AFR mode of this calculator performs all these steps automatically.
What AFR does a three-way catalytic converter need?+
A three-way catalyst (TWC) requires the engine to oscillate tightly around lambda 1.0 (AFR 14.7 for gasoline). At exactly stoichiometric conditions, the TWC simultaneously reduces NOx (by providing fuel to reduce it) and oxidises CO and HC (using residual oxygen from lean phases). Operating outside the narrow lambda 0.98 to 1.02 window for extended periods significantly reduces conversion efficiency.
What is the equivalence ratio (phi) and how does it relate to lambda?+
The equivalence ratio phi (φ) is the reciprocal of lambda: φ = 1/λ = AFR_stoich / AFR_actual. A rich mixture has φ greater than 1.0 and λ less than 1.0. A lean mixture has φ less than 1.0 and λ greater than 1.0. Aerospace and academic combustion literature typically uses φ; automotive industry standards use λ. Both convey the same information.
What is the stoichiometric AFR for hydrogen fuel?+
Hydrogen (H2) burns stoichiometrically at an AFR of 34.3 : 1. Although only 0.5 moles of O2 are needed per mole of H2 (combustion: 2H2 + O2 → 2H2O), the extremely low molar mass of hydrogen (2.016 g/mol) means a kilogram of fuel needs 34.3 kg of air. Hydrogen engines must therefore handle very large air volumes and are sensitive to pre-ignition due to hydrogen's wide flammability range (4% to 75% by volume in air).
How is AFR measured in a running engine?+
Most modern engines use a wideband lambda sensor (also called a UEGO sensor) in the exhaust. It measures oxygen partial pressure electrochemically and outputs a continuous lambda signal from about 0.65 to 2.0. Narrowband sensors only indicate rich or lean relative to stoichiometric. Dyno tuners also calculate AFR by measuring exhaust gas composition with a five-gas analyser using the Brettschneider equation.
What is the AFR for diesel engines?+
Diesel engines always run lean overall (lambda greater than 1.0, typically 1.2 to 6.0 at idle) because they rely on compression ignition and never throttle the air intake. The stoichiometric AFR for diesel is approximately 14.5 : 1. However, diesel combustion is heterogeneous — local rich zones inside the spray plume coexist with lean zones — so a global lambda above 1.0 does not eliminate all particulate matter formation.
Tags: Air-Fuel Ratio AFR Calculator Lambda Calculator Stoichiometric AFR Rich Lean Mixture Combustion Calculator Calculator Formula Free Online

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What is the air-fuel ratio (AFR)?

AFR is the mass ratio of air to fuel burned in a combustion reaction. An AFR of 14.7 : 1 means 14.7 kg of air are burned with every 1 kg of gasoline. It is the fundamental parameter controlling combustion quality, emissions, and power output in internal combustion engines.

What is the stoichiometric AFR for gasoline?

The stoichiometric AFR for gasoline is 14.7 : 1. At this ratio, all the fuel and all the oxygen are consumed simultaneously, leaving no excess of either. This is the target point for a three-way catalytic converter, which requires lambda near 1.0 to convert NOx, CO, and HC simultaneously.

What does lambda (λ) mean in engine tuning?

Lambda is the ratio of the actual AFR to the stoichiometric AFR for a given fuel: λ = AFR / AFR_stoich. A value of 1.0 is perfect stoichiometry. Values below 1.0 mean the mixture is rich (excess fuel). Values above 1.0 mean the mixture is lean (excess air).

What is a rich air-fuel mixture?

A rich mixture has more fuel than the stoichiometric ideal, so lambda is less than 1.0 and AFR is below the stoichiometric value. Rich mixtures produce more power (up to a point), lower combustion temperatures, and higher CO and HC emissions. Performance engines often target lambda 0.85 to 0.90 at full load.

What is a lean air-fuel mixture?

A lean mixture has more air than the stoichiometric ideal, so lambda is greater than 1.0. Lean mixtures improve fuel economy and reduce CO emissions but increase the risk of knock and elevated NOx at moderate lean ratios. Modern lean-burn and direct-injection engines operate at lambda 1.3 to 2.0 at light loads.

Why does ethanol need less air than gasoline?

Ethanol (C2H6O) contains oxygen in its molecular structure, so it supplies part of the oxygen needed for combustion internally. This reduces the external air requirement and lowers the stoichiometric AFR to 9.0 : 1, compared to 14.7 for gasoline. Fuel systems running on ethanol must deliver more fuel mass to reach the same lambda.

How is stoichiometric AFR calculated from a chemical formula?

For a fuel CxHyOz: the oxygen moles needed are x + y/4 - z/2. Multiply by 32 g/mol to get O2 mass, divide by 0.232 (mass fraction of O2 in air) to get air mass, then divide by the fuel molar mass. This calculator does the full computation from any CHO formula you enter.

What AFR does a catalytic converter require?

A three-way catalytic converter requires the engine to oscillate tightly around lambda 1.0 (AFR 14.7 for gasoline). The oxygen sensor feeds back to the ECU to maintain this narrow window. Running rich (lambda < 0.98) or lean (lambda > 1.02) for extended periods degrades catalyst efficiency.

What is the equivalence ratio and how does it relate to lambda?

The equivalence ratio phi equals 1 divided by lambda. A rich mixture has phi greater than 1, a lean mixture has phi less than 1, and stoichiometry is phi = 1. Some academic and aerospace references prefer phi; automotive industry references generally use lambda.

What is the stoichiometric AFR for hydrogen?

Hydrogen burns stoichiometrically at an AFR of 34.3 : 1 because hydrogen is extremely light (molar mass 2.016 g/mol) but still requires 0.5 moles of O2 per mole of H2. The resulting flame produces only water vapor, making hydrogen a zero-carbon fuel.

📌 Quick Tips

💡Use the same unit for both air mass and fuel mass — grams, kilograms, or grams per second all work, because AFR is a pure ratio.
💡Lambda (λ) of exactly 1.0 is stoichiometric. Values below 0.95 indicate a rich mixture; values above 1.05 indicate a lean mixture.
💡Ethanol and methanol have much lower stoichiometric AFR values than gasoline because they contain oxygen in their molecular structure.
💡Use Stoich AFR mode with a custom formula to find the theoretical air requirement for any pure CHO compound — useful for biofuel blend analysis.
💡For flex-fuel tuning, switch between ethanol and gasoline presets to see how the stoichiometric reference changes when calculating lambda.