The formula, in each unit
With consumption in litres per 100 km, litres used equal distance in km times consumption, divided by 100. With kilometres per litre, divide the distance by that figure instead. Miles per gallon works the same way, but a US gallon is 3.785 litres and a UK gallon 4.546 litres, so 30 mpg means a different thing in each country: about 7.8 L/100 km in US gallons and 9.4 L/100 km in UK gallons. The calculator converts everything with the exact definitions and shows your consumption in all four units under the result, so you can compare a car advert from one country with a review from another.
Find your car's real consumption
The figure on the car's brochure is measured on a test cycle, and real driving is usually thirstier. The most reliable number is your own: fill the tank to the click, reset the trip meter, drive normally until the next fill, then fill to the click again. Divide the litres you put in by the kilometres driven and multiply by 100 to get L/100 km. Most modern cars also show an average on the dashboard, which is a good second check after a few hundred kilometres of normal use.
Why the bill can come out higher
Stop-and-go traffic, short trips with a cold engine, air conditioning on full in summer heat, a loaded boot or a roof box, low tyre pressure and high motorway speeds all raise consumption. If your result is for a long summer drive with a full car, using a consumption figure a little above your average gives a more honest budget. For a daily commute, the monthly figure is most useful once you plug in the consumption you measured on that same route.
Where the price comes from
The starting price is taken from the official sources listed on our fuel price table: monthly announcements in the Gulf, the weekly UK government survey, the European Commission's weekly bulletin, and official notices in India and Pakistan, and the US Energy Information Administration's average price per US gallon. Pick the United States and the calculator switches to miles, mpg (US) and price per gallon. The note under the price field shows the date it applies from and links to the source. The table is updated by hand, not live, so if your station shows a different price, type it in; the calculation uses whatever is in the field. Nothing you enter is uploaded or saved.