Combustion Reaction Calculator
Enter the number of carbon, hydrogen, and oxygen atoms in your fuel to get the fully balanced complete-combustion equation. The calculator gives you stoichiometric coefficients, oxygen consumed, carbon dioxide and water produced (in moles and grams), the air-fuel ratio, the CO2 emission factor, and an estimated higher heating value. Works for pure hydrocarbons, alcohols, ethers, carbohydrates, and any C-H-O organic compound. Amounts update live as you type.
What is complete combustion?
Complete combustion is the reaction of a fuel with excess oxygen so that every carbon atom is converted to carbon dioxide (CO2) and every hydrogen atom becomes water (H2O). The general equation for any organic compound of formula C-alpha H-beta O-gamma is: C_a H_b O_g + n(O2) O2 -> alpha CO2 + (beta/2) H2O. Complete combustion is assumed in standard engineering and chemistry calculations, and it represents the theoretical maximum energy release from the fuel. Incomplete combustion - which occurs when oxygen is limited - produces carbon monoxide, soot, or unburned hydrocarbons instead.
How to balance a combustion equation
Balancing follows three atom-conservation rules applied in order. First, balance carbon: the number of CO2 molecules equals the number of carbon atoms in the fuel (coefficient b = alpha). Second, balance hydrogen: the number of H2O molecules equals half the hydrogen atoms (coefficient c = beta / 2). Third, balance oxygen: count the total O atoms on the right (2b + c), subtract the O atoms already in the fuel (gamma), then divide by 2 to get the O2 coefficient: n(O2) = alpha + beta/4 - gamma/2. Because this can produce a fraction (most often a half when beta is odd), chemists multiply every coefficient by 2 (or 4) to obtain the smallest set of whole numbers for publication.
Air-fuel ratio and stoichiometry
The stoichiometric air-fuel ratio (AFR) is the exact mass of air needed to burn one kilogram of fuel with no leftover oxygen and no unburned fuel. Dry air is approximately 23.2 percent oxygen by mass, so AFR = (n_O2 x M_O2) / M_fuel / 0.232. Propane, for example, needs about 15.7 kg of air per kg of fuel; methane (CH4) needs about 17.2 kg because it is hydrogen-rich relative to its molar mass. Oxygenated fuels like ethanol carry some of their own oxygen, so they need less air. Running an engine lean (excess air, AFR above stoichiometry) burns more completely but can raise nitrogen-oxide emissions; running rich (excess fuel) reduces power output and raises carbon-monoxide emissions.
Heating value and the Dulong formula
The higher heating value (HHV) is the total chemical energy released when one kilogram of fuel burns completely and all water in the products condenses back to liquid, recovering the latent heat of vaporisation. The Dulong formula, published in 1841, estimates HHV from the elemental mass fractions: HHV (MJ/kg) = 33.83 x wC + 144.3 x (wH - wO/8), where wC, wH, and wO are the mass fractions of carbon, hydrogen, and oxygen in the fuel. The (wH - wO/8) term accounts for any hydrogen already combined with fuel oxygen (e.g., in alcohols or carbohydrates). Dulong agrees with measured values to within 1-3 percent for most fossil fuels and within about 5 percent for oxygenated fuels and biomass.
Combustion properties of common fuels
| Fuel | Formula | Molar mass (g/mol) | n(O2) | AFR (kg/kg) | HHV (MJ/kg) | CO2 factor (kg/kg) |
|---|---|---|---|---|---|---|
| Hydrogen | H2 | 2.016 | 0.5 | 34.3 | 142 | 0 |
| Methane | CH4 | 16.043 | 2 | 17.2 | 55.5 | 2.743 |
| Ethane | C2H6 | 30.069 | 3.5 | 16.1 | 51.9 | 2.927 |
| Propane | C3H8 | 44.096 | 5 | 15.7 | 50.4 | 2.994 |
| Butane | C4H10 | 58.122 | 6.5 | 15.5 | 49.5 | 3.029 |
| Octane | C8H18 | 114.229 | 12.5 | 15.1 | 47.9 | 3.089 |
| Methanol | CH4O | 32.042 | 1.5 | 6.5 | 22.7 | 1.374 |
| Ethanol | C2H6O | 46.068 | 3 | 9 | 29.7 | 1.913 |
| Glucose | C6H12O6 | 180.16 | 6 | 1.06 | 15.6 | 1.467 |
Stoichiometric coefficients, AFR, HHV, and CO2 emission factors for typical fuels. Molar masses use IUPAC 2021 atomic weights.
Frequently asked questions
What inputs do I need for the combustion reaction calculator?
You need only the molecular formula of the fuel, specifically the number of carbon atoms (alpha), hydrogen atoms (beta), and oxygen atoms (gamma) per molecule. For pure hydrocarbons like methane (CH4) or propane (C3H8), set gamma to zero. For alcohols and biofuels that already contain oxygen (ethanol C2H6O has gamma = 1), enter that count. Use the Common Fuel drop-down to auto-fill these numbers for eight popular fuels.
Why are some stoichiometric coefficients given as fractions?
When the hydrogen count is not a multiple of 4, the O2 coefficient works out to a number with a one-half fraction. For example, methane (CH4) gives n(O2) = 1 + 1 - 0 = 2, which is whole; but ethane (C2H6) gives n(O2) = 2 + 1.5 - 0 = 3.5. Fractional coefficients are mathematically valid but chemists conventionally clear them by multiplying every coefficient by 2 to get 2 C2H6 + 7 O2 -> 4 CO2 + 6 H2O. This calculator shows both the unscaled rational coefficients (for amount calculations) and the scaled whole-number equation.
What is the air-fuel ratio and why does it matter?
The stoichiometric AFR is the mass of air required to burn exactly one unit mass of fuel with no leftover oxygen or unburned fuel. It matters enormously in engine design, burner calibration, and emissions control. An AFR above stoichiometry (lean mixture) leaves excess oxygen, which can reduce carbon monoxide but increase NOx. An AFR below stoichiometry (rich mixture) leaves unburned hydrocarbons and CO. Carburettors, fuel injectors, and industrial burners are all tuned to hit a target AFR.
How accurate is the Dulong formula for heating value?
For coal and liquid petroleum products the Dulong formula is accurate to within 1-3 percent of measured values. For oxygenated fuels such as ethanol or biomass it can deviate by up to 5 percent because the formula does not account for the bond energies of specific functional groups. More accurate predictions require the Boie correlation or direct calorimeter measurements. The Dulong estimate is adequate for first-pass combustion engineering and comparative analysis.
Can I use this calculator for sulfur-containing or nitrogen-containing fuels?
Not directly. This calculator handles only C, H, O compounds. Fuels that contain sulfur (S) produce SO2 in addition to CO2 and H2O, and nitrogen-containing fuels may produce NOx. To handle those, you would extend the atom-balance system with additional element equations. For coal combustion, where both S and N matter for stack-gas compliance, use a specialised coal combustion calculator or software.
What is the CO2 emission factor?
The CO2 emission factor (sometimes called the carbon intensity) expresses how many kilograms of carbon dioxide are produced per kilogram of fuel burned, assuming complete combustion. It depends solely on the carbon-to-molar-mass ratio: EF = alpha x 44.009 g/mol / M_fuel. Hydrogen (H2) has EF = 0 because it contains no carbon. Methane has EF = 2.74, ethanol 1.91, and propane 2.99. Because the emission factor is a fixed ratio of atomic weights, you cannot reduce it by making the combustion more efficient - the only way to lower CO2 per unit energy is to use a fuel with a lower carbon content or switch to hydrogen.
What does higher heating value mean compared to lower heating value?
The higher heating value (HHV) assumes that all water produced by combustion condenses to liquid, so the latent heat of vaporisation is recovered. The lower heating value (LHV) assumes the water leaves as vapour and that latent heat is not recovered. For most fuels the difference is 5-10 percent. Boilers and condensing furnaces can approach HHV efficiency by recovering latent heat from exhaust gases; internal combustion engines and gas turbines exhaust hot gases and operate on the LHV basis. The Dulong formula used here estimates HHV.