UK fuel cost calculator for a 300-mile journey

A 300-mile journey uses about 38.97 litres in a car returning 35 miles per imperial gallon, compared with 24.80 litres at 55mpg. At an illustrative petrol price of 145p per litre, the fuel-only costs would be £56.50 and £35.95 respectively. These example prices are assumptions, not current official averages.

At a glance

• Formula: miles divided by mpg, multiplied by 4.546 litres.
• A 35mpg car uses 38.97 litres over 300 miles.
• A 55mpg car uses 24.80 litres over the same distance.
• Use the newest official pump average or an actual forecourt price for your budget.

Use the newest official UK weekly fuel averages

The Department for Energy Security and Net Zero publishes the UK’s official weekly average pump prices for unleaded petrol and diesel. The supplied official information confirms the series but does not include a dated numerical observation, so no unverified August 2026 price is presented here as the latest figure.

Before calculating, open the weekly road fuel prices page and take the petrol or diesel price from the latest dated observation. Divide a price stated in pence by 100 to convert it into pounds per litre. For example, 145.0p becomes £1.45.

The weekly figure is a UK average, not a promise that a nearby filling station will charge that amount. Drivers budgeting tightly may get a more useful estimate by using the displayed price at a suitable forecourt on their planned route.

Calculate litres used and journey cost

Use miles per imperial gallon, not the smaller US gallon measure:

Litres used = journey miles ÷ mpg × 4.546

Then calculate the bill:

Journey cost = litres used × price per litre in pounds

For a 300-mile journey, the process is:

1. Choose a realistic mpg figure for the car and journey conditions.
2. Divide 300 by that mpg figure.
3. Multiply the result by 4.546 to obtain litres used.
4. Convert the pump price from pence to pounds.
5. Multiply litres used by the price in pounds.

Every price in this table is illustrative. Replace it with the newest official average or a verified local pump price. The examples also assume exactly 300 miles and steady average efficiency, with no allowance for detours, idling, parking, tolls, maintenance or vehicle depreciation.

Why the same journey can cost more in practice

National averages combine prices from many locations and retailers. They do not establish the price at any individual forecourt or prove that one type of retailer is always cheapest.

Motorway service areas may have different operating costs and less convenient competition for drivers already travelling. Supermarkets can use their own pricing and promotional strategies. Regional supply costs, local competition and the timing of price changes can also affect what appears on the sign.

Actual mpg is equally important. Congestion, cold starts, hills, weather, vehicle load and repeated acceleration can reduce efficiency. For a cautious budget, use an mpg figure that reflects previous similar journeys rather than relying automatically on an ideal laboratory or brochure value.

Reduce the bill without compromising safety

• Compare suitable nearby fuel prices before departure, but include the fuel used by any detour.
• Check tyre pressures when appropriate and follow the vehicle manufacturer’s recommended settings.
• Remove unnecessary items from the boot and take off an unused roof box or rack where practical.
• Accelerate smoothly, anticipate normal traffic flow and obey speed limits and road conditions.
• Avoid unsafe coasting, excessive speed or delaying necessary vehicle maintenance to save fuel.

Immediately before travelling, check the newest official weekly figure and the price at convenient forecourts. A change of only a few pence per litre can alter the total, while a realistic mpg estimate usually matters even more.

Source: UK Department for Energy Security and Net Zero