Fuel Cost & MPG Calculator
TrendingWork out trip fuel cost and convert between MPG and L/100km.
What is the Fuel Cost & MPG Calculator?
A fuel cost and MPG calculator works out how much a journey will cost in fuel, and converts between the UK's miles-per-gallon (MPG) and the metric litres-per-100km measure used elsewhere. It works by converting MPG into litres per 100km (roughly 282.481 ÷ MPG), converting your trip distance from miles to kilometres, then working out litres used over that distance and multiplying by the price per litre. To calculate it manually: convert MPG to L/100km using 282.481 ÷ MPG, convert your distance to kilometres (miles × 1.60934), work out litres used as (km ÷ 100) × L/100km, then multiply by the price per litre for your total cost.
How it works
Trip fuel cost is worked out from your journey distance divided by your car's fuel economy, multiplied by the current price per unit of fuel. The tricky part is that fuel economy is measured in different units depending on context — MPG (miles per gallon) is the traditional UK figure, while L/100km (litres per 100 kilometres) is the standard used across most of Europe and increasingly on newer vehicles' official figures. Because MPG is a "the bigger the better" measure and L/100km is "the smaller the better," converting between them isn't a simple like-for-like multiplication — it's an inverse relationship.
UK context
UK fuel is sold by the litre, but MPG has traditionally been quoted using the imperial gallon (which is a larger unit than the US gallon, so UK MPG figures aren't directly comparable to US MPG figures for the same car). Official manufacturer fuel economy figures come from standardised lab testing (currently the WLTP test cycle in the UK and EU), which is useful for comparing cars on equal terms but often doesn't match real-world driving, especially for shorter trips or heavy traffic.
Tips
- Work out your own real-world MPG (miles driven ÷ fuel used to cover them) for the most accurate personal fuel cost estimate, rather than relying solely on the official lab-tested figure.
- Driving style, tyre pressure, vehicle load and short journeys with a cold engine all reduce real-world fuel economy compared with the official figure.
- When comparing cars, check whether quoted figures are WLTP (current UK/EU standard) or an older test cycle, since they aren't directly comparable.
Frequently asked questions
How do I convert MPG to L/100km?
MPG and L/100km measure fuel economy in opposite directions (higher MPG is better, lower L/100km is better), so converting between them is an inverse calculation, not a simple multiplication — our calculator handles the conversion for you.
Why is my real-world MPG lower than the official figure?
Official figures come from standardised lab testing (WLTP in the UK), which doesn't fully reflect real-world conditions like stop-start traffic, cold engine starts, road gradient, or driving style — real-world MPG is very commonly somewhat lower than the official figure.
Is a UK gallon the same as a US gallon?
No — the imperial gallon used in traditional UK MPG figures is larger than the US gallon, so a UK MPG figure and a US MPG figure for the same car and same real-world economy will show different numbers. Always check which gallon a figure is based on when comparing.
A quick note
Fuel prices and vehicle economy figures change frequently — this page explains how the calculation works, using whatever price and MPG figures you enter. Always use current local fuel prices for an accurate estimate.