What next?
Fuel is the running cost. These are the rest of the numbers on a car.
How to use this calculator
- Enter the trip distance in miles or km, and your vehicle’s fuel efficiency in mpg (UK or US), L/100 km or km/L — each field carries its own unit menu, and the line under the efficiency shows the same figure in all four.
- Enter the fuel price — per litre in pence or cents, per litre in whole currency, or per UK or US gallon — and how many are sharing.
- Read the total fuel cost and cost per person.
What your result means
The total is the fuel the trip burns at your efficiency and current price; splitting between passengers shows each person’s share. It covers fuel only — not wear, tolls or parking. Better efficiency or more sharers cut the cost per head.
Why this one is different
Cost is not the only row. The volume of fuel a trip needs appears in litres and in gallons, the cost per mile is there for an expense claim, and the per-person figure divides it among everyone travelling. Miles or kilometres, mpg or litres per 100 km, and the currency are each chosen separately.
Why your commute costs more than you think
Fuel is a drip-drip expense that’s easy to ignore — until you add up a year of commuting. Knowing the true cost per mile reframes decisions about car sharing, working from home, or that scenic detour.
Split across passengers, a long drive can be cheaper than the train; alone, it can quietly cost a fortune. Seeing the number makes the choice obvious.
How it works
The calculator converts your car's miles-per-gallon into litres used for the distance, multiplies by the pump price, and — if you're car sharing — divides the total between everyone. It uses UK imperial gallons (4.546 litres).
Formula
cost = litres × price
Example calculation
A 300 mile trip at 45 mpg with fuel at £1.45/litre:
cost = 30.3 × 1.45 = £43.95
MPG, l/100km and why they do not scale the same way
Miles per gallon improves as it rises; litres per 100 km improves as it falls, and the two are reciprocals rather than a straight scale. That matters when comparing improvements: going from 20 to 25 mpg saves far more fuel over the same distance than going from 40 to 45 mpg, even though both are a five-mpg gain. In l/100km the same pair reads 14.1 to 11.3 versus 7.1 to 6.3 — the size of the saving is obvious in a way it is not in mpg. When comparing two cars, convert both to consumption per distance before judging the gap.
Real consumption is worse than the quoted figure
Official figures come from a standardised test cycle, and real-world driving is consistently thirstier. Motorway speed is the biggest single factor because drag rises with the square of speed; short urban trips are next, because a cold engine can use substantially more fuel for the first few miles. Roof boxes, low tyre pressures, air conditioning and a heavy load all add a few percent each. For budgeting, take your own figure from a full tank divided by the miles it covered rather than the manufacturer number — two or three tanks is enough to get a stable average.
Frequently asked questions
Where do I find my car's mpg?＋
Your dashboard trip computer shows real-world average mpg, which is more accurate than the manufacturer figure for everyday driving.
Is this for a return trip?＋
Enter whatever distance you want costed — double it for a return journey.
How do I convert mpg to litres per 100 km?＋
Divide 282.5 into the UK mpg figure. A car doing 45 UK mpg uses about 6.3 litres per 100 km. Note that US gallons are smaller, so US mpg figures are about 20 percent lower for the same car.
Why is my real economy worse than the official figure?＋
Official figures come from standardised tests. Short trips, cold starts, motorway speeds, roof boxes, low tyre pressure and air conditioning all cut real-world economy, often by 10 to 20 percent.
What is the cheapest speed to drive?＋
For most cars, economy peaks somewhere around 50 to 60 mph and falls away above that as aerodynamic drag rises with the square of speed. Dropping from 80 to 70 mph typically saves around 10 percent.
How do I split fuel costs fairly with passengers?＋
Work out the total fuel cost for the trip and divide by the number of people, including the driver. Some people add a share of wear and tear using the HMRC mileage rate as a reference.
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Assumptions & limitations
This is a planning estimate, not a measurement of what your car will actually use.
- Unit conversions use the exact statutory definitions: the UK gallon is 4.54609 litres, the US liquid gallon is 3.785411784 litres, and the mile is 1.609344 km. A US gallon is about 83% of a UK one, so a car quoted at 45 mpg in Britain is around 37.5 mpg in American figures — the same car, a different gallon.
- Quoted fuel economy is a laboratory figure. Real-world consumption is typically 10–20% worse, and worse still in cold weather, heavy traffic or with a full load.
- Fuel price is taken as constant for the whole journey. On a long trip the pump price will vary between regions and motorway services.
- Tolls, congestion charges, parking, servicing and tyre wear are excluded — fuel is only part of the true cost per mile.
- For an electric vehicle, enter your cost per kWh and efficiency in the same units; the arithmetic is identical but the prices behave very differently between home and rapid charging.
Sources & references
Fuel economy definitions and average pump prices are published by:
- Department for Energy Security & Net Zero — weekly road fuel prices
- Vehicle Certification Agency — official fuel consumption figures