Chemistry
Air-Fuel Ratio (AFR) Calculator
Calculate the stoichiometric air-to-fuel mass ratio for any CₓHᵧO_z fuel from its carbon, hydrogen and oxygen counts.
Your inputs
Change any value to update the result instantly.
Stoichiometric AFR (mass basis)
17.239 : 1 air : fuel
Oxygen needed per mole of fuel
2 mol O₂
Dry air needed per mole of fuel
9.5484 mol air
Molar mass of the fuel
16.043 g/mol
Actual AFR with excess air
17.239 : 1
Equivalence ratio (φ)
1
Dry air is taken as 20.946% O₂ by mole with a molar mass of 28.9647 g/mol. Complete combustion to CO₂ and H₂O is assumed.
What this calculator does
Calculates the stoichiometric air-to-fuel mass ratio for any CxHyOz fuel from the balanced complete-combustion reaction.
What your result means
The mass of dry air required to burn one unit mass of fuel completely. An equivalence ratio below 1 means a lean mixture with excess air.
Formula and working
ν(O₂) = x + y/4 − z/2 ; AFR = m_air ÷ m_fuel
- 1
Collect the inputs
Carbon atoms in the fuel (x) = 1; Hydrogen atoms in the fuel (y) = 4; Oxygen atoms in the fuel (z) = 0; Excess air supplied = 0 %
- 2
Apply the formula
ν(O₂) = x + y/4 − z/2 ; AFR = m_air ÷ m_fuel
- 3
Result
Stoichiometric AFR (mass basis) = 17.239 : 1 air : fuel
Frequently asked questions
Symbols and assumptions
- x, y, z
- Carbon, hydrogen and oxygen atoms in the fuel
- nu(O2)
- Moles of oxygen per mole of fuel = x + y/4 - z/2
- AFR
- Mass of air divided by mass of fuel
- phi
- Equivalence ratio = AFR stoichiometric / AFR actual
- • Complete combustion to carbon dioxide and water.
- • Dry air is 20.946% oxygen by mole with a mean molar mass of 28.9647 g/mol.
- • Fuel nitrogen, sulphur and moisture are not included.
Sources and review
Reviewed 2026-09-22 · Next review 2027-09-22
- • Real engines and burners run away from stoichiometric conditions; this is a design reference, not an emissions or safety calculation.
Important limitation
This is an educational estimate based only on the values and formula shown. Verify current rates, rules, units, and professional standards before relying on the result. It is not personalized financial or investment advice.
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