Combustion Air Calculator (CxHyOz fuel, excess air)

Understanding the combustion air calculator — concept and principle

Every burner, boiler and engine needs a known amount of air to burn its fuel completely. This calculator takes a fuel written as CxHyOz, adds the excess air you choose, and returns the oxygen demand, the air per mole of fuel, the mass air-fuel ratio and the expected O₂ and CO₂ in the dry flue gas.

Each O₂ brings about 3.77 N₂ along, so nitrogen fills most of the flue gas.

How it is calculated

Complete combustion: CxHyOz + (x + y/4 − z/2) O₂ → x CO₂ + (y/2) H₂O.
Stoichiometric oxygen: O₂ = x + y/4 − z/2 (mol per mol fuel).
Actual air: air = O₂ ÷ 0.2095 × (1 + EA/100), because dry air is 20.95% O₂ by volume.
Mass air-fuel ratio: air × 28.965 ÷ (12.011x + 1.008y + 15.999z), using molar masses in g/mol.
Dry flue gas = CO₂ (x) + nitrogen and argon (air × 0.7905) + unused O₂ (O₂ × EA/100); the O₂ and CO₂ percentages are each part divided by this total.

The principle

Each carbon atom needs one O₂ to become CO₂, every four hydrogen atoms need one O₂ to become two H₂O, and oxygen already in the fuel reduces the demand. Air brings 3.77 moles of inert gas with every mole of O₂, so the inert part dominates the flue gas. Real burners cannot mix fuel and air perfectly, so a little excess air is supplied to avoid carbon monoxide and soot. Too much excess air, however, heats extra nitrogen and oxygen that leave through the stack, wasting fuel.

Worked example

Default fuel is methane, CH₄: x = 1, y = 4, z = 0, with 15% excess air.
O₂ = 1 + 4/4 − 0 = 2 mol/mol fuel.
Air = 2 ÷ 0.2095 × 1.15 = 10.98 mol/mol fuel (9.55 without excess air).
Fuel molar mass = 12.011 + 4 × 1.008 = 16.043 g/mol, so AFR = 10.9785 × 28.965 ÷ 16.043 = 19.82 (the stoichiometric value is 17.24).
Dry flue gas = 1 CO₂ + 8.679 N₂/Ar + 0.3 O₂ = 9.979 mol.
O₂ = 0.3 ÷ 9.979 = 3.01%, CO₂ = 1 ÷ 9.979 = 10.02%.

Each point is the calculator re-run with only “Oxygen atoms z” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Oxygen atoms z	Stoichiometric O₂ (mol/mol fuel)
0	2
1	1.5
2	1
3	0.5
4	0
5	-0.5
6	-1

How to read the result

For methane at zero excess air the dry flue gas would contain about 11.7% CO₂ and no O₂. As excess air rises, O₂ rises and CO₂ falls, so either reading tells you how lean the flame is. Gas firing usually runs with less excess air than oil or solid fuels because gas mixes with air more easily.

Common mistakes

Assuming air is 21% O₂ and 79% N₂ and then comparing with precise analyser data; small rounding changes the result.
Comparing these dry-basis percentages with a wet-basis measurement.
Forgetting that oxygen in the fuel (alcohols, biomass) lowers the air needed.

Keep in mind: The model assumes complete combustion, dry air and a pure single fuel. Sulphur, nitrogen, ash, humidity and CO are ignored. Burner settings must follow the manufacturer's procedure and local safety codes.

Frequently asked questions

What is the stoichiometric air-fuel ratio of methane?

About 17.2 kg of air per kg of methane, which matches step 4 without excess air.

How do I enter a fuel blend?

Use average atom counts weighted by mole fraction; for example, a gas of 90% CH₄ and 10% C₂H₆ gives x = 1.1 and y = 4.2.

Why does the flue gas CO₂ never reach 100%?

Nitrogen from the air always makes up most of the dry flue gas.

Sources: Turns, S. R. An Introduction to Combustion: Concepts and Applications. McGraw-Hill. · Perry's Chemical Engineers' Handbook, combustion section. McGraw-Hill.

Checked 8 Nov 2026. Standard textbook formulas in SI units, computed in your browser. For learning and estimates — real designs must follow your local codes and be checked by a qualified engineer.

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