Hong Life CalculatorsUnderstanding the fuel CO₂ emissions calculator — concept and principle
This calculator estimates the carbon dioxide released when you burn a given amount of petrol, diesel, LPG, natural gas or jet fuel. It is useful for a personal carbon footprint, comparing vehicles, estimating the emissions of a trip or preparing a simple company fuel inventory.
How it is calculated
- CO₂ (kg) = fuel amount × emission factor
- Tonnes = kg ÷ 1000
- Tree equivalent = kg ÷ 21, a rough illustration assuming one tree absorbs about 21 kg of CO₂ per year.
Built-in factors: petrol 2.31 kg/L, diesel 2.68 kg/L, LPG 1.51 kg/L, natural gas 2.02 kg/m³, jet fuel 2.52 kg/L. Liquids are entered in litres and natural gas in cubic metres.
The principle
Almost all the carbon in a fuel ends up as CO₂ when it burns. Each carbon atom (12 g/mol) combines with two oxygen atoms from the air to form CO₂ (44 g/mol), so every kilogram of carbon becomes about 3.67 kg of CO₂. That is why the CO₂ mass can exceed the mass of the fuel itself. A litre of petrol weighs roughly 0.74 kg and is about 85% carbon, giving roughly 0.63 kg of carbon and therefore about 2.3 kg of CO₂. Diesel is denser and contains more carbon per litre, so its factor is higher. These are "tank-to-wheel" combustion factors: they do not include emissions from extracting, refining and transporting the fuel.
Worked example
- Inputs: petrol, 100 L.
- CO₂ = 100 × 2.31 = 231 kg.
- In tonnes: 231 ÷ 1000 = 0.231 t.
- Tree equivalent: 231 ÷ 21 = 11 trees for one year.
| Amount (L or m³) | CO₂ (kg) |
|---|---|
| 25 | 57.75 |
| 50 | 115.5 |
| 75 | 173.25 |
| 100 | 231 |
| 125 | 288.75 |
| 150 | 346.5 |
| 200 | 462 |
How to read the result
A car using 7 L of petrol per 100 km emits about 16 kg of CO₂ per 100 km from combustion. Compare options per unit of useful output, such as per kilometre or per kWh of heat, rather than per litre, because fuels differ in energy content: a litre of LPG emits less CO₂ than a litre of petrol, but it also contains less energy, so you need more of it. The tree figure is only a picture of scale; real absorption varies widely with species, age and climate.
Common mistakes
- Entering kilograms of fuel instead of litres, or kWh of gas instead of m³.
- Treating combustion CO₂ as the full life-cycle footprint; well-to-wheel values are higher.
- Assuming tree planting cancels emissions immediately; young trees absorb little.
Frequently asked questions
Why does burning 1 kg of fuel produce more than 1 kg of CO₂?
Because most of the CO₂ mass comes from oxygen taken from the air, not from the fuel.
Does an electric car have zero emissions?
It has no tailpipe CO₂, but the electricity has its own emissions that depend on the grid mix; use an electricity CO₂ calculator for that part.
Are biofuels counted the same way?
Reporting rules usually record biogenic CO₂ separately because the carbon was recently absorbed by plants; check the method your programme uses.
Sources: IPCC 2006 Guidelines for National Greenhouse Gas Inventories, Volume 2 (Energy), Chapter 3 (Mobile Combustion) · GHG Protocol, Corporate Accounting and Reporting Standard (Scope 1 direct emissions)