UK bank-holiday fuel costs: calculate your journey

A reliable August bank-holiday fuel budget needs the latest weekly UK pump-price averages and your car’s real-world efficiency. The Department for Energy Security and Net Zero publishes the official weekly series, but the confirmed information available here does not include the latest observation date, current prices or previous week’s figures. Quoting a cash total without those values would be misleading.

By GlobeBids Money Desk | Published 19 August 2026

• Use the newest completed DESNZ observation available when you calculate.
• Record petrol and diesel in pence per litre, including the previous week’s values.
• Treat national averages as budgeting inputs, not guaranteed forecourt prices.
• Check your route and nearby fuel options before entering a motorway.

Copy the latest petrol and diesel figures before calculating

Open the DESNZ weekly road-fuel prices page and find the most recent completed observation available on the day of travel. Record five items: the observation date, current petrol average, previous petrol average, current diesel average and previous diesel average.

Those numerical fields are not present in the confirmed facts accompanying this article, so they cannot be reproduced responsibly here. Once verified, a mobile-friendly comparison can show the latest and previous observations without implying that every driver will pay the national average.

The official series reports UK average pump prices. It cannot establish which local filling station is cheapest, and it does not guarantee the price at a motorway service area. RAC Fuel Watch provides additional consumer-facing context on pump-price movements, but the DESNZ observation should remain the consistent input for this calculation.

Use this journey-cost formula

UK fuel economy is commonly expressed in miles per imperial gallon, while pump prices are quoted per litre. One imperial gallon equals 4.54609 litres, so the conversion matters.

1. Divide journey miles by your real-world miles per gallon.
2. Multiply the result by 4.54609 to obtain litres used.
3. Multiply litres by the verified price in pence per litre.
4. Divide by 100 to express the result in pounds.

The complete formula is:

Journey cost (£) = (miles ÷ real-world mpg) × 4.54609 × (price in pence per litre ÷ 100)

Use the full figures during the calculation and round only the final total to the nearest penny. For a return journey, include every planned mile rather than doubling an approximate outward distance if the routes differ.

Set up a representative 250-mile journey

For a car returning 45mpg in normal use, a 250-mile journey would consume:

250 ÷ 45 × 4.54609 = 25.26 litres

After inserting the latest verified DESNZ prices, calculate the two totals as follows:

• Petrol cost: 25.26 × verified petrol price ÷ 100
• Diesel cost: 25.26 × verified diesel price ÷ 100

This deliberately stops short of publishing a cash result because the necessary current prices and observation date are absent from the confirmed information. It also assumes equal efficiency for illustration. A diesel vehicle and a petrol vehicle may achieve different real-world mpg, so each should normally be calculated using its own figure.

Why the amount paid can differ from the estimate

National averages smooth out substantial differences between individual forecourts. Motorway sites may charge differently from stations near towns, while strong local competition can affect nearby prices. A pump may also change its price after the weekly observation was collected.

Vehicle load, speed, weather, air-conditioning use and driving style all influence consumption. Bank-holiday congestion can increase fuel use through slow traffic and repeated acceleration. Roadworks, missed turns and discretionary detours add miles that were not included in the original estimate.

Your car’s displayed economy may also differ from its long-term measured result. For a cautious budget, use an mpg figure from comparable journeys and allow a modest contingency rather than assuming the vehicle’s best recorded efficiency.

Reduce spending without adding road risk

• Compare convenient fuel options before joining a motorway, when it is safe to do so.
• Check tyre pressures when the tyres are cold and follow the vehicle manufacturer’s guidance.
• Combine errands so that several short trips become one planned journey.
• Remove unnecessary weight, but keep all safety and emergency equipment you need.
• Check traffic information before departure and include any sensible diversion in the mileage.

Never make an unsafe turn, stop in an unsuitable place or take a lengthy unfamiliar diversion merely to save a small amount at the pump. Extra miles can erase the saving, while safety must take priority over a marginal price difference.

The final check is the newest DESNZ observation and the actual prices on or near the planned route. Those figures turn the formula into a useful budget; the UK average alone cannot identify the cheapest local or motorway forecourt.

Source: Department for Energy Security and Net Zero