Fuel cost calculator
Work out what a journey costs in petrol or diesel, and whether a detour to a cheaper forecourt is worth it. Prices default to today's UK average from official Fuel Finder data.
Journey cost
Is the detour to a cheaper station worth it?
Uses the cheaper price for the fuel burned on the detour. Ignores your time and wear and tear.
How the fuel cost calculator works
Fuel used (litres) = miles ÷ mpg × 4.546, or km × L/100km ÷ 100. Cost = litres × price per litre. UK fuel economy figures use the imperial gallon (4.546 litres), so make sure you enter UK mpg, not US mpg.
For the detour check we compare the saving on your fill-up (litres × price difference) with the cost of the extra fuel the detour burns. The break-even distance tells you how far you could drive before the cheaper forecourt stops saving you money. Find the cheapest forecourts near you on the LitreDrop home page or see today's UK fuel prices report.
FAQ
How much does a litre of petrol cost in the UK today?
The UK average for petrol (E10) is 174.1p a litre and diesel is 199.2p, based on official Fuel Finder data updated 15 Oct 2026, 16:38 BST.
How do I work out the fuel cost of a journey?
Divide the distance by your car's miles per gallon to get gallons used, multiply by 4.546 to convert UK gallons to litres, then multiply by the price per litre. The calculator does this for you.
What is the difference between UK and US mpg?
A UK (imperial) gallon is 4.546 litres and a US gallon is 3.785 litres, so the same car shows a higher mpg figure in UK gallons. Use UK mpg here, as quoted by UK manufacturers and on your dashboard.
How do I convert mpg to L/100km?
L/100km = 282.48 ÷ UK mpg. For example 45 mpg is about 6.3 L/100km.
Is it worth driving further for cheaper fuel?
Only if the saving on the fill-up is more than the fuel burned on the extra miles. Saving = litres bought × price difference. Detour cost = extra miles ÷ mpg × 4.546 × price. For a 50-litre fill a 5p difference saves £2.50, which covers roughly 15 extra miles in a 45 mpg car.
Does a half-empty tank change the answer?
Yes. The saving depends on how many litres you actually buy, not the tank size, so a top-up rarely justifies a long detour.