Planning a road trip? Work out the fuel cost before you leave so you don’t get caught short. You only need three things: how far you’re going, how thirsty your car is, and what fuel costs today. Use our free Fuel Cost Calculator and you’ll know exactly how much petrol or diesel to budget for any route. The numbers are simple. Put the distance, your car’s official MPG figure, and the current pence-per-litre price into the formula and you’ve got an accurate cost in minutes. It’s not guesswork—just plug and play.
Understanding fuel cost road trip
Start with three numbers: the distance you’ll drive, the car’s official combined MPG, and the current fuel price. Imagine a 300-mile trip in a car that does 45 MPG with petrol at £1.50 per litre. First convert MPG to miles per litre: a gallon is 4.55 litres, so 45 ÷ 4.55 gives about 10.3 miles per litre. Divide 300 miles by 10.3 and you’ll need roughly 29 litres. Multiply that by £1.50 and the fuel cost lands at about £43.50.
Use the manufacturer’s combined figure, not the city-cycle number. That combined figure already includes motorway and urban driving, so it’s the most reliable one to use.
Plug the distance, the MPG, and the pence-per-litre price into the formula. Litres needed = distance ÷ (MPG ÷ 4.55). Cost = litres × price per litre.
How to work it out, step by step
- Enter your trip distance in kilometres.
- Enter your vehicle efficiency in km per litre.
- Enter the fuel price per litre.
- See the fuel used and total cost.
Fuel used = Distance ÷ Efficiency · Cost = Fuel used × PriceWhy it matters
Misjudge the fuel and you could be walking to the nearest garage with an empty tank. Overestimate the MPG or forget a detour and a £10 shortfall on a 300-mile trip can easily become £50 if traffic or extra miles eat into your reserve. Build in a 5–10% buffer on litres—especially on routes you don’t know. A scenic diversion or unexpected queue can swallow extra miles faster than you think.
Free tools for this
| Tool | What it does |
|---|---|
| Fuel Cost Calculator | Work out the fuel needed and total cost for any trip. |
| Unit Price Calculator | Find the price per unit so you can spot the best value. |
Try the Fuel Cost Calculator
Skip the manual maths — enter your numbers and get the answer instantly.
Open the Fuel Cost Calculator →Good to know
Bookmark this page so the tool is only a tap away next time you need a quick, reliable answer. It works on your phone just as well as on a computer, with no app to install.
Key takeaways
- Use your car’s combined MPG figure, not the urban one.
- Add 5–10% extra fuel to cover detours and traffic.
- Check the pump price in pence per litre, not per gallon.
Frequently asked questions
What units does this take?
Kilometres for distance, kilometres per litre for efficiency, and price per litre. There is no mpg or L/100km input, so convert your figure first — entering mpg into the km/L box will give you a badly wrong answer rather than an error.
How do I convert mpg or L/100km to km/L?
Multiply US mpg by roughly 0.425, or imperial mpg by roughly 0.354. For L/100km, divide 100 by it — 6 L/100km is about 16.7 km/L. Note that L/100km is inverted: a lower number is a more efficient car.
Why do US and UK mpg figures differ for the same car?
Because the gallons differ. An imperial gallon is about 4.55 litres and a US gallon about 3.79, so the same car returns a bigger number in imperial mpg — around 20% higher — despite burning identical fuel. It is a units artefact, not a better engine.
Where do I find my real fuel efficiency?
Fill the tank, reset the trip meter, drive normally, then fill again and divide the kilometres driven by the litres it took to refill. That gives your true km/L, which is usually worse than the manufacturer's quoted figure and far better for budgeting.
How accurate is the cost?
It is exact arithmetic on the numbers you supply, but real-world consumption varies with speed, load, terrain, weather and driving style. Treat it as a budgeting estimate — motorway cruising and stop-start city traffic can differ enormously for the same car.
