Combustion Air-Fuel Ratio Calculator
Determine the air-to-fuel mass ratio needed for complete combustion of methane, propane, gasoline, or diesel. Use this when tuning burners, engines, or industrial furnaces to optimize efficiency and minimize emissions.
About this calculator
The air-fuel ratio (AFR) is the mass of air required to completely combust a unit mass of fuel. Each fuel has a stoichiometric AFR: methane ≈ 17.2, propane ≈ 15.7, gasoline ≈ 14.7, and diesel ≈ 14.5. Running with excess air prevents incomplete combustion and carbon monoxide formation, but too much excess air reduces flame temperature and efficiency. The actual AFR is calculated as: AFR = AFR_stoich × (1 + excessAir / 100). For example, 20% excess air on gasoline gives AFR = 14.7 × 1.20 = 17.64. The air mass flow rate is then: ṁ_air = AFR × ṁ_fuel. Knowing AFR helps engineers balance combustion quality, fuel economy, and pollutant output across boilers, gas turbines, and internal combustion engines.
How to use
Suppose you are burning propane with 15% excess air at a fuel mass flow rate of 0.5 kg/s. Step 1 — Select fuel type: propane, stoichiometric AFR = 15.7. Step 2 — Enter excess air: 15%. Step 3 — Calculate actual AFR: AFR = 15.7 × (1 + 15/100) = 15.7 × 1.15 = 18.055. Step 4 — Enter fuel mass flow: 0.5 kg/s. Step 5 — Air mass flow: ṁ_air = 18.055 × 0.5 = 9.03 kg/s. You now know you need approximately 9 kg of air per second to combust 0.5 kg/s of propane at 15% excess air.
Frequently asked questions
What is the stoichiometric air-fuel ratio and why does it matter?
The stoichiometric AFR is the theoretically perfect ratio of air to fuel that allows complete combustion with no leftover oxygen or unburned fuel. For gasoline it is approximately 14.7:1, meaning 14.7 kg of air per 1 kg of fuel. Operating at stoichiometry maximizes energy release per unit of fuel. Deviating rich (less air) produces CO and unburned hydrocarbons, while running lean (more air) can cause misfires or NOₓ formation.
How does excess air percentage affect combustion efficiency?
Excess air is the amount of air supplied above the stoichiometric requirement, expressed as a percentage. A small positive excess (5–20%) ensures complete fuel burnout and guards against local fuel-rich zones that produce soot and CO. However, too much excess air dilutes the flame, lowering peak temperature and increasing heat losses through the exhaust. Optimal excess air varies by application: gas boilers typically run at 10–15%, while large industrial furnaces may use 20–30%.
Why do different fuels have different stoichiometric air-fuel ratios?
The stoichiometric AFR depends on the carbon-to-hydrogen ratio and oxygen content of the fuel molecule. Fuels with more hydrogen per carbon atom (like methane, CH₄) need proportionally more oxygen and therefore more air, giving methane an AFR of about 17.2. Heavier hydrocarbons like diesel have a lower hydrogen fraction, reducing their AFR to about 14.5. This difference is why a carburetor or fuel injector calibrated for one fuel will run incorrectly on another without recalibration.