About the Fuel Economy Calculator
Fuel Economy Calculator does two jobs for drivers. First, it converts fuel economy between miles per US gallon, miles per UK gallon, litres per 100 kilometres and kilometres per litre, so you can compare cars from different countries. Second, it works out what a trip costs: enter the distance in kilometres or miles, the fuel price per litre or gallon and the number of people sharing, and it shows the fuel used, the total cost, the cost per kilometre and per mile, each person's share and the carbon dioxide produced. Add a tank size to see your range. A second mode works out your real fuel economy from the distance driven and the fuel you put in. Everything runs in your browser.
Updated September 2026
How to use the Fuel Economy Calculator
- 1Choose Convert and trip cost or Work out my economy.
- 2Enter your fuel economy and pick its unit, for example 7.5 litres per 100 km or 32 mpg.
- 3Enter the trip distance, the fuel price and its unit, and the number of people sharing.
- 4Optional: add your tank size to see the range on a full tank.
- 5Read the economy in every unit, the trip fuel, cost, share per person and CO₂.
Three ways to say the same thing
Fuel economy is measured in different ways around the world. The United States and the United Kingdom use miles per gallon, but their gallons are different sizes. Most of Europe, Canada and Australia use litres per 100 kilometres, while parts of Asia use kilometres per litre. When you look at a car made for another market, or a review that quotes another unit, you need a quick way to compare.
The tool converts between all four at once. Type a figure in the unit you know and read the rest.
Planning a trip
- Enter the distance in kilometres or miles and the fuel price per litre or gallon.
- Add the number of people sharing to get each person's share.
- Use your own measured economy for the most realistic number.
- Add 10 to 20 percent for traffic, hills, luggage and air conditioning.
- Add the tank size to see how far a full tank should take you.
Measuring your own economy
The best figure is the one you measure. Fill the tank to the click, reset the trip meter, drive as usual and fill again at the same pump. The litres you add and the distance on the meter give your real economy. Repeat over a few tanks and average them, because a single tank can vary with weather and driving style.
Use the Work out my economy mode to enter the distance and the fuel added, in the units you have. The result appears in all four units and can be used above for trip costs.
Related tools
Unit Converter changes distances, volumes and temperatures, Percentage Calculator works out price changes, Tip Calculator splits shared bills, Energy Cost Calculator shows the running cost of appliances and Loan Calculator helps plan a car purchase.
Features
- US mpg, UK mpg, L/100 km and km/L conversions
- Trip fuel, cost, cost per km and per mile
- Share of the cost per person
- Range from your tank size
- Real economy from distance and fuel used
- Price per litre, US gallon or UK gallon
- CO₂ for petrol or diesel
- Runs in your browser
Frequently asked questions
How do I convert mpg to litres per 100 km?
Divide 235.2 by the miles per gallon if it is a US gallon figure, or 282.5 if it is a UK gallon. So 30 US mpg is 235.2 ÷ 30 = 7.84 litres per 100 km. The tool does this for you and shows all units at once.
Why are US and UK mpg different?
A UK, or imperial, gallon is 4.546 litres and a US gallon is 3.785 litres, about 20 percent smaller. The same car therefore has a bigger mpg number in the UK than in the US. 30 US mpg equals about 36 UK mpg.
Is a lower or higher number better?
It depends on the unit. Miles per gallon and kilometres per litre measure distance per fuel, so higher is better. Litres per 100 km measures fuel per distance, so lower is better.
How do I calculate the fuel cost of a trip?
Multiply the distance by the litres per 100 km and divide by 100 to get the litres, then multiply by the price per litre. A 300 km trip at 7.5 L/100 km uses 22.5 litres, and at 1.80 per litre the cost is 40.50. The tool handles miles and gallons too.
How do I work out my real fuel economy?
Fill the tank, note the distance on the trip meter, then fill up again and note the litres. Divide the litres by the distance and multiply by 100 to get litres per 100 km. Choose Work out my economy and enter the two numbers, and the tool converts to every unit.
How accurate is the trip cost?
It is as accurate as the economy figure you enter. Manufacturers' figures are measured in test conditions, and real driving is usually 10 to 30 percent worse in city traffic, at high speeds, with a heavy load or in cold weather. Use your own measured economy for the best estimate.
How is the CO₂ figure worked out?
Burning a litre of petrol produces about 2.31 kilograms of carbon dioxide and a litre of diesel about 2.68 kilograms. The tool multiplies your trip fuel by these numbers. It covers only exhaust emissions, not the energy used to produce and deliver the fuel.