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Combustion & Air–Fuel Ratio Calculator

For engines, boilers, furnaces and emissions estimates. Enter a fuel formula (or pick one) and optionally the excess air your burner runs with.

Fuels:

2C8H18 + 25O2 → 16CO2 + 18H2O

O₂ per mol fuel
12.5 mol
Stoichiometric AFR
15.13 : 1
AFR with excess air
15.13 : 1
Air per kg fuel
11.71 m³ (0 °C)
CO₂ per kg fuel
3.082 kg
H₂O per kg fuel
1.419 kg
  1. Fuel C8H18O0: M = 114.232 g/mol
  2. O₂ needed = C + H/4 + S − O/2 = 8 + 18/4 + 0 − 0/2 = 12.5 mol
  3. Air = O₂ ÷ 0.2095 = 59.67 mol × 28.965 g/mol ÷ 114.23 g/mol = AFR 15.13

Assumes complete combustion in dry air (20.95 % O₂ by volume). Nitrogen in the fuel is assumed to leave as N₂; real flames also make some NOₓ and, if air is short, carbon monoxide and soot.

How it works

For complete combustion, carbon becomes CO₂, hydrogen becomes H₂O, sulfur becomes SO₂ and fuel nitrogen is assumed to leave as N₂. The oxygen needed per mole of fuel is C + H/4 + S − O/2. Dry air is 20.95 % oxygen by volume with a molar mass of 28.965 g/mol, which gives the air needed and the mass air–fuel ratio.

The stoichiometric air–fuel ratio for petrol (modelled as octane) is about 15.1 : 1 by mass, and for natural gas (methane) about 17.2 : 1. Real burners add excess air to make sure all the fuel burns, at the cost of some heat carried away in the flue gas.

Frequently asked questions

What is the air–fuel ratio of petrol?
About 14.7 : 1 for real petrol blends and 15.1 : 1 for pure octane. It is the mass of air needed to burn one mass unit of fuel completely.
How much CO₂ does burning 1 kg of methane make?
CH₄ + 2O₂ → CO₂ + 2H₂O: 44.01 ÷ 16.04 = 2.74 kg of CO₂ per kg of methane.
Why use excess air?
Mixing is never perfect. A little extra air prevents unburned fuel, carbon monoxide and soot. Too much wastes energy heating air that goes up the chimney.

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