The fuel-cost formula
The whole calculation is three steps. Total distance (doubled for a return trip) sets how far you drive; consumption converts distance into litres burned; the pump price turns litres into money:
litres = distance(km) × consumption(L/100km) ÷ 100
cost = litres × price per litre
Everything else the tool shows — cost per person, CO₂ — is derived from that litre figure. The litres of fuel, not the kilometres, is the quantity worth thinking about.
Consumption is the variable that matters
Distance and price are roughly fixed by your route and the day you fill up. Consumption is where vehicles differ enormously, and it is expressed two ways: litres per 100 km (lower is better, used in most of the world) and miles per gallon (higher is better). For the same 500 km trip at €1.80/L:
| Vehicle | L/100 km | Fuel for 500 km |
|---|---|---|
| Efficient hybrid | 4.5 | 22.5 L · ≈ €40 |
| Small petrol car | 6.5 | 32.5 L · ≈ €59 |
| Mid-size estate | 8.0 | 40 L · ≈ €72 |
| Large SUV | 11.0 | 55 L · ≈ €99 |
Why your real consumption beats the brochure
Use your own observed figure, not the manufacturer's. Official type-approval numbers (WLTP, and the older NEDC) are measured in a lab and real-world driving typically runs 14–24% higher (ICCT, 2022). Motorway speed, cold weather, roof boxes, a full car and aggressive acceleration all push it up further. Your trip computer's long-term average is the best input.
The CO₂ figure
Burning a litre of petrol releases about 2.31 kg of CO₂ at the tailpipe (DESNZ, 2024); diesel is denser and emits ~2.68 kg/L. This is a direct chemical consequence of the carbon in the fuel, so it scales exactly with litres burned — the only way to cut it is to burn less. Note this is the tailpipe figure; counting extraction and refining (well-to-wheel) adds roughly another 20%.
Splitting the cost
Cost per person is simply the fuel cost divided by the number of travellers — the calculation behind splitting fuel fairly on a shared trip. Four people in one efficient car can move for a few euros each; the same four in separate cars multiply both cost and emissions fourfold.
What this does not include
This estimates fuel only. The true cost of driving is far higher once you count tolls, parking, tyre and brake wear, servicing, insurance and depreciation — full cost-of-ownership studies put the all-in figure at several times the fuel cost per kilometre (ADAC, 2024). For splitting money with friends, fuel-only is the fair and customary basis; for deciding whether to drive at all versus train or fly, compare full costs. The estimate also assumes steady consumption — stop-start city traffic and mountain passes will exceed it.