Fuel Cost Calculator

What Is a Fuel Cost Calculator?

Petrol and diesel prices fluctuate weekly, and the cost of running a car is one of the expenses most people underestimate when budgeting. Whether you're planning a long drive, comparing two vehicles, or trying to work out the annual fuel bill on a potential car purchase, knowing the actual numbers makes the decision considerably clearer.

At 140p per litre — roughly mid-2025 average for unleaded in the UK — a 400-mile round trip in a car averaging 40mpg costs around £45 in fuel. The same trip in a car averaging 30mpg costs £60. Over a year of average UK driving (around 7,400 miles), that efficiency difference adds up to roughly £600. The type of driving matters too: motorway miles at a steady speed are significantly more efficient than urban stop-start traffic.

Enter the trip distance, your car's fuel economy (in mpg or L/100km), and the current fuel price per litre. You'll get the estimated fuel cost for the journey and, if you enter your annual mileage, a full-year running cost projection.

How Do You Use This Fuel Cost Calculator?

Enter the trip distance, your vehicle's fuel efficiency (km/L, L/100km, or mpg), and the current fuel price per litre or gallon. Click Calculate to see the fuel required and total cost.

1. Enter the total distance of your journey in kilometres or miles.
2. Input your vehicle's fuel efficiency (km/L, L/100km, or miles per gallon).
3. Enter the current fuel price per litre or per gallon.
4. Click Calculate to see the fuel required and total cost.
5. Review the results and adjust distance or efficiency to compare scenarios.
6. Use the results to budget for your trip or compare vehicles.

How Does the Fuel Cost Calculator Formula Work?

The formula used: Fuel Needed = Distance / Fuel Efficiency; Total Cost = Fuel Needed × Price per Litre (or Gallon)

What Are Some Example Calculations?

A 300 km trip in a car that does 15 km/L with fuel at £1.45/L: 300 / 15 = 20 litres needed. Cost = 20 × £1.45 = £29.00.

When Should You Use a Fuel Cost Calculator?

Before a long journey — particularly when comparing the cost of driving versus rail. Factor in parking, tolls, and your actual mpg (not the manufacturer's claimed figure, which is typically 15–20% optimistic) for a realistic comparison.

When buying a used car. The difference in fuel costs between a 35mpg and a 50mpg car over 50,000 miles of ownership at 140p/litre is around £3,000. That figure is rarely front of mind when evaluating purchase price, but it's a meaningful part of the total cost of ownership.

What Do These Terms Mean?

What Are the Best Tips to Know?

• Check your car's actual fuel efficiency from trip computer data rather than relying on the manufacturer's claimed figure.
• Factor in a 10-15% buffer for city driving, air conditioning, or hilly terrain which all increase consumption.
• Compare fuel costs between petrol, diesel, and electric to evaluate whether switching vehicle type saves money.
• Use real-time fuel price apps to find the cheapest station along your route before setting off.
• For monthly budgeting, multiply your daily commute cost by 22 working days rather than 30 calendar days.

What Mistakes Should You Avoid?

• Using the manufacturer's official fuel economy figure instead of real-world consumption, which is typically 10-20% higher.
• Forgetting to account for the return journey when calculating a round trip.
• Mixing up L/100km (lower is better) with km/L (higher is better) — they work in opposite directions.
• Not adjusting for driving conditions: motorway cruising is more efficient than stop-start city traffic.