Free AFR Calculator

The air-fuel ratio (AFR) is a fundamental parameter in any combustion process, from internal combustion engines to industrial burners. This AFR Calculator determines the mass of air required for the complete combustion of a given mass of fuel, expressed as a ratio of air mass to fuel mass.

Understanding Air-Fuel Ratio (AFR)

The AFR indicates how much air (oxygen) must be supplied to achieve stoichiometric combustion—where all fuel is burned with no excess oxygen. It is most commonly expressed on a mass basis:

$$\text{AFR} = \frac{m_{\text{air}}}{m_{\text{fuel}}}$$

The same concept can be expressed on a molar basis ($\overline{\text{AFR}}$) by relating the number of moles of air and fuel, taking into account their respective molar masses.

Knowing the AFR is essential for designing efficient burners, tuning engines, and minimizing emissions. The stoichiometric AFR varies with fuel composition; richer or leaner mixtures lead to incomplete combustion or wasted energy.

Air-Fuel Ratio for Common Fuels

Different fuels contain varying proportions of carbon and hydrogen, resulting in different oxygen requirements. The following table lists the stoichiometric AFR (mass and molar) for several common hydrocarbon fuels, along with their molar masses.

Fuel	Formula	AFR (mass)	AFR (molar)	Molar mass (g/mol)
Methane	CH4	17.19	9.52	16.04
Ethane	C2H6	16.95	16.66	30.07
Propane	C3H8	15.64	23.80	44.09
Butane	C4H10	15.42	30.94	58.12
Pentane	C5H12	15.29	38.08	72.15
Octane	C8H18	15.09	59.50	114.23
Diesel (approx.)	C12H23	14.6	84.49	167.31
Hydrogen	H2	34.21	2.38	2.02

For example, gasoline (represented by octane) requires about 15.09 kg of air per kg of fuel, while diesel fuel requires roughly 14.6 kg of air per kg of fuel. Natural gas, primarily methane, has an AFR of 17.19:1. Hydrogen, with its high energy content, demands a much higher air mass per unit mass.

Calculating the Stoichiometric Air-Fuel Ratio

The theoretical (stoichiometric) air is the minimum amount of air needed for complete combustion. For a generic hydrocarbon fuel with the formula $C_\alpha H_\beta$, the balanced combustion reaction with theoretical air is:

$$C_\alpha H_\beta + a\left(O_2 + 3.76 N_2\right) \rightarrow b CO_2 + c H_2O + d N_2$$

The coefficients are determined by the carbon and hydrogen atom counts:

• $b = \alpha$
• $c = \frac{\beta}{2}$
• $a = \alpha + \frac{\beta}{4}$
• $d = 3.76 \times a = 3.76 \left(\alpha + \frac{\beta}{4}\right)$

Once these coefficients are known, the molar AFR ($\overline{\text{AFR}}$) is simply the ratio of moles of air (a) to moles of fuel (1). The mass AFR is then obtained by multiplying the molar ratio by the molar masses:

$$\text{AFR} = \overline{\text{AFR}} \times \frac{M_{\text{air}}}{M_{\text{fuel}}}$$

where $M_{\text{air}} \approx 28.97\ \text{g/mol}$ and $M_{\text{fuel}}$ is the fuel's molar mass.

This method can be applied to any fuel with a known molecular formula. If the formula is not immediately available, a combustion analysis can first determine the elemental composition.

How to Use the AFR Calculator

Using this tool is straightforward:

1. Select a fuel from the provided list (e.g., methane, octane, diesel, or hydrogen). The calculator will instantly display the stoichiometric AFR for that fuel.
2. If you have a specific amount of fuel or air, enter the known mass. The calculator will compute the required mass of the other substance to achieve stoichiometric combustion.
3. For custom fuels, choose "Other" and enter the masses of air and fuel; the calculator will then compute the resulting AFR.

By knowing the stoichiometric AFR, you can also derive the lambda value ($\lambda$), which is the ratio of actual AFR to stoichiometric AFR. Lambda is widely used in engine tuning to indicate whether the mixture is lean (λ > 1) or rich (λ < 1). With the stoichiometric AFR as a baseline, a lambda calculator can quickly assess the mixture quality for performance and emissions control.