Combustion Air Calculator: Air Needed to Burn a Fuel, Excess Air, CO2 and Water Produced

How much air a fuel needs, and what comes out.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

Choose the fuel.

Enter the excess air as a percent.

The calculator gives the air needed and the products of combustion.

Formulas used

  • C_aH_bO_c + \left(a + \frac{b}{4} - \frac{c}{2}\right)O_2 \to aCO_2 + \frac{b}{2}H_2O
  • AFR = \frac{n_{O_2}M_{O_2}}{0.2314\,M_{fuel}}

Worked example

With the default numbers in the calculator above:

  1. Oxygen needed per mole of fuel: 2 mol
  2. Air needed for complete burning (theoretical), by mass: 17.239 kg per kg of fuel
  3. Air supplied with your excess air: 18.963 kg per kg of fuel
  4. Carbon dioxide produced: 2.7432 kg per kg of fuel
  5. Water produced: 2.2458 kg per kg of fuel
  6. Molar mass of the fuel: 16.043 g/mol

Change the numbers in the calculator to see the full working, every step, for your own case.

A second worked example

A different case, with these numbers: Fuel Propane (C3H8); Excess air 20 %.

  1. Oxygen needed per mole of fuel: 5 mol
  2. Air needed for complete burning (theoretical), by mass: 15.679 kg per kg of fuel
  3. Air supplied with your excess air: 18.815 kg per kg of fuel
  4. Carbon dioxide produced: 2.994 kg per kg of fuel
  5. Water produced: 1.6341 kg per kg of fuel
  6. Molar mass of the fuel: 44.097 g/mol

Common questions

What is the air-fuel ratio of methane?

About 17.2 kg of air for each kg of methane for exact combustion.

What is excess air?

The extra air supplied beyond the exact amount, to make sure all the fuel burns. It is given as a percent of the theoretical air.

How much CO2 does a fuel make?

Methane makes 2.74 kg of CO2 for each kg burned, propane 3.0 kg and carbon 3.66 kg.

Related calculators

Got a question about this? Ask an engineering question and get a plain-language answer with references.

Further reading: Any thermodynamics or combustion text (stoichiometric combustion).

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