Fuel Efficiency Calculator
Convert your car's fuel efficiency between MPG (UK), L/100km, and km/L. UK MPG uses the imperial gallon (4.54609 litres). Also calculate fuel used and cost for a journey.
Convert fuel efficiency
Enter a fuel efficiency value and select its unit to see conversions to all other units.
Related: MPG Calculator · Fuel Cost
Calculate fuel used and cost
Enter your MPG, miles driven, and fuel price to calculate fuel consumption and trip cost.
How the Fuel Efficiency Calculator works
This calculator converts between the three most common fuel efficiency units: UK MPG (miles per gallon using the 4.54609-litre imperial gallon), L/100km (litres per 100 kilometres, the European standard), and km/L (kilometres per litre). Enter any one value, select its unit, and the calculator instantly produces the other two. This is particularly useful when comparing a UK-spec car with a European model whose data sheet quotes L/100km.
The second section lets you calculate fuel used and cost for a specific journey. Enter your MPG, the distance, and the pump price in pence per litre to see how many litres your car will consume and what it will cost. This is a quick way to budget before a long journey or to compare the running costs of two vehicles you are considering buying.
Bear in mind that official UK MPG figures quoted by manufacturers are based on WLTP laboratory testing and may not reflect your real-world driving. Actual consumption depends on speed, load, road conditions, climate, and driving style. Using your own measured MPG — calculated from fill-up to fill-up — will give the most accurate results.
Frequently asked questions
What is a fuel-efficient car in the UK?
In the UK, a fuel-efficient petrol or diesel car typically achieves 50 MPG or more on a combined cycle. Many modern hybrid vehicles return 60–80 MPG, while smaller superminis and city cars commonly achieve 45–55 MPG. Electric vehicles have no MPG figure but have significantly lower per-mile running costs, typically equivalent to 200+ MPG in energy terms.
How is fuel efficiency measured in the UK?
Fuel efficiency in the UK is officially measured in miles per gallon (MPG) using the imperial gallon (4.54609 litres). New cars are tested under the WLTP (Worldwide Harmonised Light Vehicle Test Procedure) standard, which replaced the older NEDC test. Real-world figures are typically 10–20% lower than official figures due to driving conditions, load, and temperature.
Does driving style affect fuel efficiency?
Yes, driving style is one of the biggest factors affecting real-world fuel efficiency. Smooth acceleration and gentle braking, maintaining a steady speed, anticipating traffic flow, and avoiding unnecessary idling can all improve efficiency by 10–20%. Aggressive driving — harsh braking, rapid acceleration, and high motorway speeds — can reduce MPG significantly compared to gentler driving.
How do I improve my car's fuel efficiency?
To improve fuel efficiency, keep tyres inflated to the manufacturer's recommended pressure (under-inflation increases rolling resistance), remove unused roof racks or boxes, avoid carrying excess weight, use the correct grade of engine oil, and ensure your car is regularly serviced. Driving in the highest appropriate gear and using cruise control on motorways also helps maintain consistent, economical speeds.