Free · Stoichiometry calculators 🧪

Air-to-Fuel Mass Ratio & Fuel Presets

Relate air mass, fuel mass and their ratio with fixed fuel presets or a custom air-to-fuel ratio.

  • Formula & worked example
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Calculator inputs

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How to use this calculator

  1. Enter the known values in the units shown. Results update as you type.
  2. Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
  3. Use the worked example to check the method. Reset restores the starting fields.

Private by default

Inputs and results stay in this browser tab. Bookify does not upload or store the values you enter.

Formula and method

A custom air-to-fuel mass ratio of 15:1 requires 15 kg of air for 1 kg of fuel. The same ratio holds when both masses use pounds.

Air-to-fuel mass ratio = air mass / fuel mass

Worked example

Enter these known values and leave the other values blank.

Fuel preset or custom ratio
Custom mass ratio
Fuel mass
1 kg
Air mass
15 kg
Custom air-to-fuel mass ratio
15 :1

Assumptions and limitations

  • Masses and ratios are positive. AFR is a dimensionless mass ratio, not a volume ratio; the numerator is air rather than pure oxygen.
  • The fourteen fuel presets are fixed approximate stoichiometric ratios. Actual values depend on fuel composition and the oxygen content of the air, particularly for commercial blends.
  • Custom mode accepts your own ratio or calculates it from measured masses. A preset is not a recommended operating mixture or an engine-performance prediction.

Common questions

Is a preset the best ratio for every engine?

No. It represents an illustrative stoichiometric mass ratio. Engine operating requirements involve additional conditions that this mass balance does not model.

Can I mix grams and kilograms?

Yes. Select the unit for each mass; the calculation converts them before taking the ratio.

References

The calculation equations, inverse formulas, units, and input rules were imported from this source. Bookify provides the interface and equation solver.

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