The method
Divide the distance by your car's miles per gallon to get the gallons used. Multiply by 4.546 to turn UK gallons into litres. Multiply the litres by the price per litre. For a 420-mile trip in a car doing 48 miles per gallon, with fuel at 136.9p a litre, that is 8.75 gallons, or 39.8 litres, costing about £54.46.
UK and US gallons are different
A UK (imperial) gallon is 4.546 litres. A US gallon is only 3.785 litres. The same car shows a higher mpg figure in UK gallons than in US gallons, so figures from American websites are not directly comparable. The fuel cost calculator lets you choose which one your figure uses.
Use real-world economy
Official economy figures come from standard tests and are often better than everyday driving. Your car's trip computer, or a brim-to-brim check with the MPG calculator, gives a more realistic figure. Motorway speed, roof boxes, cold weather and short trips all use more fuel.
Sharing the cost
If friends share a journey, divide the total fuel cost by the number of people. Remember to double the distance for a return trip.