Fuel Cost Calculator
What a journey costs in fuel, split between passengers — with return trips and route comparison.
Runs entirely in your browser. Nothing you type here is uploaded, stored or logged.
How the calculation works
Two steps. Divide distance by economy to find the fuel consumed, then multiply by the price. Everything else is unit conversion — which is exactly where mental arithmetic tends to fail, because fuel is priced per litre in most of the world while economy is quoted per gallon in the UK and US.
A worked example: 300 miles at 40 mpg (UK) uses 7.5 UK gallons. A UK gallon is 4.546 litres, so that's 34.1 litres. At £1.45 a litre the trip costs about £49.44.
The three economy units, and why they don't mix
| Unit | Used in | Better is |
|---|---|---|
| mpg (UK) | United Kingdom | Higher |
| mpg (US) | United States | Higher |
| L/100km | Europe, Canada, Australia | Lower |
| km/L | India, Japan, Brazil, much of Asia | Higher |
The UK and US gallons differ: a UK gallon is 4.546 litres, a US gallon 3.785. The same car reads about 20% better in UK mpg than US mpg, which makes cross-Atlantic comparisons of "40 mpg" meaningless without knowing which gallon.
L/100km is inverted relative to the others, and it's the more honest unit for comparing savings. Going from 10 to 9 L/100km saves as much fuel over a fixed distance as going from 5 to 4.5 — but in mpg terms those look like very different improvements.
Why official figures overstate real economy
Manufacturer figures come from a standardised test cycle in controlled conditions. Real driving adds everything the lab excludes:
- Cold starts — the first few miles can be 30–50% worse while the engine warms.
- Speed. Aerodynamic drag rises with the square of speed. 80 mph can use 15–20% more fuel than 70.
- Air conditioning — typically 5–10% in hot weather.
- Roof boxes and bike racks — an empty roof box can cost 10–20% on a motorway.
- Tyre pressure. Under-inflation of 20% costs roughly 3% in economy, and wears tyres faster.
- Weight. Roughly 1–2% per 50 kg carried.
A 10–20% shortfall against the official figure is normal. If you want an accurate trip cost, measure your own.
Measuring your real economy
- Fill the tank completely and note the odometer or reset the trip meter.
- Drive normally — don't change your habits for the test.
- Fill completely again at the same pump if possible, and record the litres added.
- Divide the distance by the fuel added.
- Repeat over two or three tanks and take the average.
This is more reliable than the dashboard computer, which is commonly optimistic by 3–8%.
Splitting the cost fairly
For a shared journey, fuel is the obvious cost but not the only one. Tyres, servicing and depreciation are real, which is why mileage allowances — around 45p per mile in the UK, 67 cents in the US — are far above pure fuel cost.
For a lift with friends, splitting fuel is the normal courtesy and this tool gives you the number. For work travel, claim the official mileage rate instead; it's meant to cover the whole cost of running the car.
Frequently asked questions
How do I work out the fuel cost of a trip?
Divide the distance by your fuel economy to get the fuel used, then multiply by the price per unit. For 300 miles at 40 mpg: 7.5 gallons, and at £1.45 a litre (£6.59 a gallon) that is about £49. The tool handles the unit conversions, which is where most manual attempts go wrong.
What is the difference between mpg, L/100km and km/L?
mpg and km/L measure distance per unit of fuel — higher is better. L/100km measures fuel per fixed distance — lower is better. They are inversely related, which is why you cannot average them the same way. Also note UK and US gallons differ by about 20%, so a UK mpg figure looks better than the same car's US figure.
Why is my real economy worse than the official figure?
Official figures come from standardised laboratory cycles. Real-world driving adds cold starts, hills, air conditioning, roof boxes, headwinds, traffic and tyre pressure. A gap of 10–20% is normal, and short urban journeys can be far worse. Use your own measured figure if you have one.
How do I measure my actual fuel economy?
Fill the tank completely, reset the trip meter, drive normally for a few hundred miles, then fill completely again. Divide the distance by the fuel added. Repeat over two or three tanks and average — a single measurement is noisy. This beats the dashboard readout, which is often optimistic by several percent.
Does the calculator work for electric vehicles?
Indirectly. Enter your efficiency in miles per kWh as the economy figure and your electricity price per kWh as the fuel price, and the arithmetic is identical. Bear in mind home and public charging prices differ enormously.