What this calculator does
Complete combustion of a CHO compound always gives carbon dioxide and water, so balancing only needs bookkeeping. Every carbon becomes one CO₂, every two hydrogens become one H₂O, and the oxygen requirement follows.
Whether the equation needs a coefficient on the fuel depends on whether the oxygen comes out whole. Propane needs exactly 5 O₂ and balances directly; ethane needs 3.5, so the whole equation doubles to give 2C₂H₆ + 7 O₂. That is the only complication in the whole procedure.
The formula
Carbon dioxide equals the carbon count and water equals half the hydrogen count. Oxygen required is carbon plus a quarter of the hydrogen, minus half any oxygen already in the fuel, since that oxygen contributes to the products. If the result is fractional, everything is multiplied up to the smallest whole numbers.
| Term | Meaning |
|---|---|
| Complete combustion | Enough oxygen to give only CO₂ and water. Incomplete combustion gives carbon monoxide and soot. |
| Stoichiometric | Exactly the oxygen required, no more and no less. |
| Oxygenated fuel | One containing oxygen, such as ethanol, which needs less added oxygen per carbon. |
| Coefficient | The number in front of a species in the balanced equation. |
The inputs explained
| Field | What to enter |
|---|---|
| Carbon atoms (α) | Carbon atoms in the fuel molecule. |
| Hydrogen atoms (β) | Hydrogen atoms. |
| Oxygen atoms (γ) | Oxygen atoms in the fuel. Zero for a pure hydrocarbon. |
When to use it
Balancing a combustion equation
The standard exercise, and one where the fractional oxygen case trips people up.
Calculating oxygen demand
How much oxygen a given fuel quantity needs, for burner and engine design.
Comparing fuels
Oxygenated fuels such as ethanol need less air per unit of fuel, which changes engine tuning.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What does each hydrocarbon need?
A range of carbon counts with hydrogen fixed.
| Carbon atoms | Balanced equation | O2 required per mole of fuel | CO2 produced per mole of fuel |
|---|---|---|---|
| C = 1 | CH8 + 3 O2 → CO2 + 4 H2O | 3.000 | 1.000 |
| C = 2 | C2H8 + 4 O2 → 2 CO2 + 4 H2O | 4.000 | 2.000 |
| C = 3 | C3H8 + 5 O2 → 3 CO2 + 4 H2O | 5.000 | 3.000 |
| C = 4 | C4H8 + 6 O2 → 4 CO2 + 4 H2O | 6.000 | 4.000 |
Questions
Why do some equations need a coefficient on the fuel?
Because the oxygen requirement comes out fractional. Ethane needs 3.5 O₂ per molecule, and equations are conventionally written in whole numbers, so everything doubles to 2C₂H₆ + 7 O₂. Propane needs exactly 5 and requires no doubling.
What about incomplete combustion?
It produces carbon monoxide and soot alongside CO₂ and water, and has no single balanced equation because the product mix depends on how much oxygen is available. This calculator handles complete combustion only, which is the stoichiometric reference case.
How does oxygen in the fuel change things?
It reduces the external oxygen needed, by half an O₂ per oxygen atom in the fuel. Ethanol C₂H₆O needs 3 O₂ where ethane C₂H₆ needs 3.5. This is why ethanol carries less energy per litre and why engines running it need more fuel per unit of air.
Does this work for fuels containing nitrogen or sulfur?
No. Those elements produce their own oxides, NOx and SO₂, which are not accounted for here. The calculation covers carbon, hydrogen and oxygen only, which is the great majority of ordinary fuels.
For the air required by mass, see the air-fuel ratio calculator. For finding a formula from combustion products, see the combustion analysis calculator.