Calculate the balanced combustion equation, oxygen required, CO₂ and H₂O produced, heat released, and air-to-fuel ratio for common fuels and any CxHyOz compound. Choose from preset fuels or enter your own hydrocarbon formula.
✓ Verified: NIST WebBook thermochemical data & IUPAC stoichiometry — April 2026
Common Fuel Presets
Number of C atoms per moleculeEnter carbon count (1 or more).
Number of H atoms per moleculeEnter hydrogen count (0 or more).
Number of O atoms (0 for pure HC)Enter oxygen count (0 or more).
Enter to calculate product masses
Optional — shown in equation label
Balanced Equation
—
💡
Was this calculator helpful?
✓ Thanks for your feedback!
Sources & Methodology
🛡️Stoichiometric coefficients derived from standard combustion balancing equations. Heat of combustion values from NIST WebBook standard thermochemical data (298 K, 1 atm).
🔬
NIST Chemistry WebBook — Standard Enthalpies of Combustion
Authoritative thermochemical data for heats of combustion at standard conditions. webbook.nist.gov
Standard treatment of enthalpy of combustion, Hess's law, and stoichiometric calculations for combustion reactions.
⚛️
IUPAC — Quantities, Units and Symbols in Physical Chemistry (Green Book)
Standard conventions for enthalpy notation and combustion reaction balancing. iupac.org
Balancing Formula for CxHyOz + O₂ → CO₂ + H₂O:
O₂ coefficient = x + y/4 − z/2
CO₂ coefficient = x
H₂O coefficient = y/2
If any coefficient is fractional → multiply all by 2 to get whole numbers
Molar mass fuel = 12.011x + 1.008y + 15.999z
A combustion reaction is a rapid exothermic reaction between a fuel and an oxidant — almost always oxygen — that releases energy in the form of heat and light. For organic fuels containing carbon, hydrogen, and sometimes oxygen, complete combustion produces only carbon dioxide and water. The energy released is what powers engines, furnaces, gas cookers, and power plants worldwide.
Balancing combustion reactions is a fundamental stoichiometry skill. The general formula for complete combustion of any hydrocarbon or oxygenated compound CxHyOz is: CxHyOz + (x + y/4 − z/2)O₂ → x CO₂ + (y/2) H₂O. When the oxygen coefficient is fractional, multiply the entire equation by 2 to get whole-number coefficients.
Common Fuel Combustion Equations
Fuel
Formula
Balanced Equation
ΔH° (kJ/mol)
Methane (natural gas)
CH₄
CH₄ + 2O₂ → CO₂ + 2H₂O
−890
Ethane
C₂H₆
2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O
−1,560
Propane (LPG)
C₃H₈
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
−2,220
Butane (lighter fluid)
C₄H₁₀
2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O
−2,878
Octane (gasoline proxy)
C₈H₁₈
2C₈H₁₈ + 25O₂ → 16CO₂ + 18H₂O
−5,471
Ethanol
C₂H₅OH
C₂H₆O + 3O₂ → 2CO₂ + 3H₂O
−1,366
Hydrogen
H₂
2H₂ + O₂ → 2H₂O
−286/mol H₂
Calculating the Air-to-Fuel Ratio
The stoichiometric air-to-fuel ratio (AFR) is the exact mass of air required per unit mass of fuel for complete combustion with no excess oxygen. Since air is approximately 23.2% oxygen by mass, the oxygen mass needed is divided by 0.232 to get the air mass required.
For methane: 1 mol CH₄ (16.04 g) requires 2 mol O₂ (64.00 g). AFR = 64.00 / (16.04 × 0.232) = 17.2 kg air per kg methane. For octane (gasoline): AFR ≈ 14.7:1, which is why 14.7 is the target lambda for gasoline engines.
💡 Complete vs Incomplete Combustion:
Complete combustion (excess O₂): Only CO₂ + H₂O produced. Maximum energy released.
Incomplete combustion (insufficient O₂): CO + soot + unburned HC also produced. Less energy released. Toxic CO produced.
Rich mixture (more fuel than stoichiometric) → incomplete combustion.
Lean mixture (more air than stoichiometric) → complete combustion but higher NOx.
Heat of Combustion and Energy Density
The heat of combustion (ΔH°ₒ) is the enthalpy change when one mole of a substance undergoes complete combustion at standard conditions (25°C, 1 bar). It is always negative (exothermic). The higher heating value (HHV) includes the heat recovered by condensing the water vapour in the products; the lower heating value (LHV) does not. For engine and turbine calculations, LHV is usually used because exhaust water exits as steam.
How Combustion Powers Engines
In an internal combustion engine, the fuel-air mixture is compressed and ignited. The rapid combustion creates high-pressure gases that push the piston down. The efficiency of this process depends critically on the air-fuel ratio: too rich wastes fuel and produces CO, too lean can cause misfires or increase NOx emissions. Modern engines use oxygen sensors and engine control units (ECUs) to maintain near-stoichiometric combustion across operating conditions.
Frequently Asked Questions
A combustion reaction is a rapid exothermic chemical reaction between a fuel and an oxidant (usually oxygen) that produces heat and light. In complete combustion of an organic compound, carbon forms CO₂ and hydrogen forms H₂O. Combustion is the fundamental reaction behind most energy production from fossil fuels and biomass.
For CxHy: CxHy + (x + y/4) O₂ → x CO₂ + (y/2) H₂O. For CxHyOz: CxHyOz + (x + y/4 − z/2) O₂ → x CO₂ + (y/2) H₂O. If the O₂ coefficient is fractional, multiply the entire equation by 2 to get whole-number coefficients.
In complete combustion of a hydrocarbon or organic compound, the only products are carbon dioxide (CO₂) and water (H₂O). Incomplete combustion also produces carbon monoxide (CO), soot (elemental carbon), and unburned hydrocarbons when insufficient oxygen is present.
Methane combustion: CH₄ + 2O₂ → CO₂ + 2H₂O. Two moles of O₂ per mole of CH₄. In mass terms: 16 g methane requires 64 g oxygen (ratio 1:4). In volume at STP: 1 L methane requires 2 L oxygen (or ~9.5 L air since air is ~21% O₂).
The heat of combustion is the energy released when one mole of substance completely burns in oxygen at standard conditions (25°C, 1 atm). It is always negative (exothermic). Key values: methane −890 kJ/mol, propane −2,220 kJ/mol, octane −5,471 kJ/mol, ethanol −1,366 kJ/mol, hydrogen −286 kJ/mol.
Complete combustion occurs when sufficient oxygen is present — all carbon becomes CO₂ and all hydrogen becomes H₂O. Incomplete combustion (oxygen-limited) also produces carbon monoxide (CO), soot, and unburned hydrocarbons. Incomplete combustion releases less energy and produces toxic CO. The air-to-fuel ratio determines which occurs.
For CxHyOz: 1) Put coefficient 1 on the fuel. 2) Balance C: put x in front of CO₂. 3) Balance H: put y/2 in front of H₂O. 4) Balance O: put (x + y/4 − z/2) in front of O₂. 5) If any coefficient is fractional, multiply all by 2 to get whole numbers.
The stoichiometric AFR is the exact mass of air needed for complete combustion per unit mass of fuel. For gasoline (octane C₈H₁₈) it is ~14.7:1; for methane ~17.2:1; for ethanol ~9.0:1. Running rich (AFR below stoichiometric) wastes fuel. Running lean (AFR above stoichiometric) is more efficient but can increase NOx emissions.
Ethanol (C₂H₆O) already contains oxygen in its molecular structure and has a lower carbon-to-hydrogen ratio than octane. Per gram of fuel burned, ethanol produces ~1.91 g CO₂ while octane produces ~3.09 g CO₂. This is one reason ethanol blends reduce vehicular CO₂ emissions per kilometre driven.