Skip to content
ENExplore Calculators

Combustion Air Requirement Calculator

Work out combustion air requirement instantly with clear inputs, formula shown and shareable results.

About Combustion Air Requirement Calculator

Complete combustion of CxHy needs x + y/4 moles of oxygen, and since dry air is 20.95% oxygen by mole, the air requirement is that divided by 0.2095. Converting both sides to mass with 28.96 g/mol for air gives the air-fuel ratio: octane comes out near 15.1:1, the familiar petrol figure. Excess air scales the requirement without changing the stoichiometry.

Formulas

Oxygen requirement

O2 = x + y/4 per mole of fuel

Air-fuel ratio by mass

AFR = (O2 / 0.2095) x 28.96 / (12.011x + 1.008y)

Worked example

With the default values below, Combustion Air Requirement Calculator returns Stoichiometric Air-Fuel Ratio of 15.129. Change any input above and every figure here is recalculated in your browser.

Inputs used

Carbon atoms per fuel moleculeminimum 1
8
Hydrogen atoms per fuel moleculeminimum 1
18
Excess air suppliedaccepts 0–300 %
20 %

Results

Stoichiometric Air-Fuel Ratio
15.129 kg air/kg fuel
Air-Fuel Ratio With Excess Air
18.155 kg air/kg fuel
Oxygen Needed Per Mole Of Fuel
12.500 mol
Air Needed Per Mole Of Fuel
59.666 mol

Step by step

  1. 1Values used
  2. 2Oxygen requirement
  3. 3Air-fuel ratio by mass
  4. 4Stoichiometric Air-Fuel Ratio= 15.129 kg air/kg fuel
  5. 5Air-Fuel Ratio With Excess Air= 18.155 kg air/kg fuel
  6. 6Oxygen Needed Per Mole Of Fuel= 12.500 mol
  7. 7Air Needed Per Mole Of Fuel= 59.666 mol

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why is excess air used in practice?

Mixing is never perfect, so a stoichiometric charge leaves unburnt fuel. Boilers typically run 10-20% excess to complete combustion without wasting too much heat up the flue.

Does this include the nitrogen?

Yes, in the air figure - the 0.2095 division brings the accompanying nitrogen along, which is what carries heat out of the exhaust.

Related Calculators

Combustion Air Requirement Calculator | Calcrivo