AFR Calculator (Air-Fuel Ratio)

How to use this air-fuel ratio calculator

What this calculator does

This calculator estimates the stoichiometric air-fuel ratio on a mass basis: the theoretical mass of ordinary dry air required for complete combustion of one unit mass of fuel. It also converts between a chosen fuel mass and its corresponding theoretical air mass. The result is a chemistry reference point, not a burner setting, engine-tuning recommendation, emissions guarantee, or substitute for equipment-specific testing. Actual systems often operate with excess air, enrichment, exhaust-gas recirculation, or transient controls.

When to use it

Use it to check combustion homework, size a first-pass theoretical air requirement, compare how different fuels change air demand, or prepare a transparent calculation sheet for laboratory and engineering discussions. It is also useful when checking whether a quoted air or fuel mass is consistent with a stated AFR.

How to calculate

1. Choose a fuel in Select a fuel. The built-in list supplies its stoichiometric AFR.
2. Choose the desired Mass unit. Existing mass values are converted automatically.
3. Enter Mass of fuel to calculate Mass of air, or edit Mass of air to solve backward for fuel mass.
4. For a custom substance or measured ratio, select Other / custom AFR and enter the desired Air-fuel ratio (AFR).
5. Read the ratio, required air, total mixture mass, and fuel share in Live results. Use Download Excel to export the current typed values and outputs, or Reset to restore methane, kilograms, 1 kg fuel, and 17.19 kg air.

Input guide

Select a fuel is required and accepts one named fuel or the custom option. Choosing methane, for example, sets AFR to 17.19. A common mistake is treating a listed hydrocarbon surrogate – especially diesel – as an exact property of every commercial blend. Air-fuel ratio (AFR) is a positive decimal in kilograms of air per kilogram of fuel. It is locked for built-in fuels and editable for Other; 14.70 is a realistic custom example. Zero, negatives, scientific notation, and comma-decimal values are rejected. Raising AFR increases required air for the same fuel mass. Mass unit is required and may be kg, g, or lb; changing it converts both masses, while AFR stays dimensionless. Mass of fuel and Mass of air accept nonnegative U.S.-style decimal numbers without unit text. Enter 2.5, not “2,5 kg.” Editing either mass makes it the driving value for the next calculation.

Output guide

Stoichiometric air-fuel ratio is the primary identity in the form air:fuel. Fuel mass and Required air mass reproduce the currently solved pair in the selected unit. Total mixture mass is their sum before combustion. Fuel share of mixture equals fuel mass divided by total reactant mass; it falls as AFR rises. A zero fuel or air mass produces a zero mixture and a 0% share rather than an undefined display. The summary pills repeat the selected fuel, AFR, and mass basis. The Common fuel reference table shows each built-in ratio, its approximate fuel share, and the air needed for 1 kg of fuel.

Worked example

With methane selected and 1.000 kg entered as fuel mass, the calculator uses AFR = 17.19. Required air is 1.000 × 17.19 = 17.190 kg. Total mixture mass is 1.000 + 17.190 = 18.190 kg. Fuel share is 1.000 ÷ 18.190 × 100 = 5.50%. The displayed ratio is therefore 17.19:1.

Learn more

The U.S. Department of Energy guide to checking burner air-to-fuel ratios explains why practical equipment often uses excess combustion air. For molecular identity and molecular-weight data, see the NIST Chemistry WebBook.