Formula
Fuel used = distance ÷ fuel economyCost = fuel used × price per unit of fuelWith L/100 km: fuel (L) = distance (km) × L/100 km ÷ 100 How it works
Trip fuel cost comes down to two numbers: how much fuel the journey burns and what that fuel costs. Divide the distance by your fuel economy to get gallons or litres, then multiply by the pump price.
Use a realistic fuel economy. Window-sticker figures are measured under gentle test cycles; sustained highway speeds above 65 mph, roof boxes, towing, heavy loads and air-conditioning can cut real-world mileage by 10–25%. Your car’s trip computer or a fill-up-to-fill-up calculation is the best source.
You can mix units freely — miles with L/100 km, or kilometres with a price per US gallon. Everything is converted internally, and the fuel needed can be shown in US gallons, litres or imperial gallons.
Frequently asked questions
What is the difference between US and UK mpg?
A UK (imperial) gallon is 4.546 L, about 20% larger than a US gallon of 3.785 L, so the same car shows a higher mpg figure in the UK. Pick the matching unit to avoid underestimating the cost.
How do I convert mpg to L/100 km?
Divide 235.21 by the US mpg figure. 25 mpg is about 9.4 L/100 km.
Does this include tolls or wear and tear?
No, only fuel. The IRS mileage rate is a common benchmark for the full per-mile cost of running a car.