Step 01
Use matching units.
UK miles per gallon uses the imperial gallon, equal to approximately 4.54609 litres. It is not the smaller US gallon. For UK MPG, divide miles by MPG to obtain imperial gallons, then multiply by 4.54609 for litres.
For litres per 100 km, multiply distance in kilometres by the entered efficiency and divide by 100. Once fuel use is in litres, multiply it by the pump price per litre.
- UK MPG litres = miles divided by MPG, multiplied by 4.54609.
- L/100 km litres = kilometres multiplied by L/100 km, divided by 100.
- Journey cost = litres used multiplied by price per litre.
- Cost per traveller = complete journey cost divided by people sharing.
Step 02
Worked example: 150 miles at 45 UK MPG.
A 150-mile journey at 45 UK MPG uses about 15.15 litres. At £1.45 per litre, the estimated journey cost is £21.97, or about 14.65p per mile.
If three people split that complete cost equally, the fuel-only estimate is about £7.32 each. A return journey should be entered as two trips if the distance field contains one direction.
Step 03
Choose a realistic efficiency figure.
Dashboard averages from similar journeys are usually more useful than a best-case manufacturer figure. Traffic, cold starts, speed, weather, roof boxes, towing and vehicle load can all change actual consumption.
Use the current local pump price when making a purchase or reimbursement decision. Published UK averages are useful for a broad comparison but will not match every region or filling station.
Step 04
Fuel cost is not total vehicle cost.
The result excludes insurance, depreciation, servicing, tyres, parking, tolls and finance. For business travel, do not assume a fuel estimate is the same as an allowable mileage claim; use the relevant employer and HMRC rules.
Official UK weekly benchmark
Average petrol and diesel pump prices.
Week commencing 19 October 2026. Use a current local forecourt price for an individual journey.
Latest six published weeks
| Week commencing | Petrol | Diesel |
|---|---|---|
| 19 October 2026 | 174.57p/L | 199.33p/L |
| 12 October 2026 | 175.28p/L | 200.04p/L |
| 5 October 2026 | 174.88p/L | 199.52p/L |
| 28 September 2026 | 173.46p/L | 197.58p/L |
| 21 September 2026 | 172.01p/L | 195.53p/L |
| 14 September 2026 | 168.14p/L | 190.72p/L |
What the change means
An illustrative 100-mile journey at 45 UK MPG.
That journey uses about 10.10 litres. Applying the same efficiency to each benchmark isolates the effect of pump-price changes.
Illustration only: real fuel use varies by vehicle, route, load, traffic, weather and driving style.
Source: Weekly road fuel prices, Department for Energy Security and Net Zero. These are national weekly averages, not a forecast or a local quote.
Contains public sector information licensed under the Open Government Licence v3.0.
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Keep a practical worksheet.
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Journey fuel-cost worksheet
Compare routes or vehicles using consistent distance, efficiency, price and traveller inputs.
Sources and further guidance
References used for this guide.
Weekly road fuel prices
Department for Energy Security and Net Zero on GOV.UK. Accredited official statistics for average UK petrol and diesel pump prices, updated weekly.