Fuel Consumption Calculator

About the Fuel Consumption Calculator

Fuel economy is described in more ways than almost any other everyday figure. In most of Europe, Asia and Australia, cars are rated in litres per 100 kilometres. In parts of Asia and Latin America, kilometres per litre is common. In the United States it is miles per US gallon, and in the United Kingdom miles per imperial gallon — a different, larger gallon. Reading a review from another country, importing a car or simply moving abroad means translating between them, and because one of these measures is upside down compared with the others, the conversion is not a simple multiplication.

This fuel consumption calculator does two jobs. From a trip, enter the distance you drove and the fuel you used — in kilometres and litres, or miles and US or UK gallons — and it works out your consumption. To convert a figure, enter a fuel economy in any of the four units and it gives all the others. Either way you get L/100 km, km per litre, US mpg and UK mpg, with a plain rating of how economical the figure is.

How to Use the Fuel Consumption Calculator

Choose from a trip or convert a figure.

For a trip, enter the distance driven and the fuel used, and choose the units they are in.

To convert, enter the fuel economy and choose the unit it is given in.

How Fuel Consumption Is Worked Out

  L/100 km   =  litres ÷ km × 100
  km/L       =  100 ÷ L/100 km
  US mpg     =  235.215 ÷ L/100 km
  UK mpg     =  282.481 ÷ L/100 km

  1 mile       =  1.609 km
  1 US gallon  =  3.785 litres
  1 UK gallon  =  4.546 litres

The constants 235.215 and 282.481 come from 100 km per mile-per-gallon: 100 × 3.785 ÷ 1.609 for the US gallon, and 100 × 4.546 ÷ 1.609 for the UK gallon.

Step-by-Step Example

A trip of 450 km using 36 litres.

  L/100 km:   36 ÷ 450 × 100           =  8
  km/L:       100 ÷ 8                  =  12.5
  US mpg:     235.215 ÷ 8              =  29.4
  UK mpg:     282.481 ÷ 8              =  35.3

Converting 30 US mpg.

  L/100 km:   235.215 ÷ 30             =  7.84
  km/L:       100 ÷ 7.84               =  12.8
  UK mpg:     282.481 ÷ 7.84           =  36

Conversion Table

Why L/100 km and mpg Behave Differently

Litres per 100 km measures fuel used for a fixed distance, so a lower number is better. Miles per gallon and km per litre measure distance for a fixed amount of fuel, so a higher number is better. Because one is the reciprocal of the other, equal steps in mpg do not mean equal savings. Improving a car from 10 to 20 mpg saves far more fuel than improving another from 30 to 40 mpg, even though both are 10 mpg better.

Over 1,000 miles, the first improvement cuts fuel use from 100 gallons to 50, a saving of 50 gallons. The second cuts it from 33.3 gallons to 25, a saving of just 8.3 gallons. Litres per 100 km shows this directly, which is one reason many economists prefer it, and why the US fuel economy label now also shows gallons per 100 miles.

US and UK Gallons

The imperial gallon used in the UK is 4.546 litres; the US gallon is 3.785 litres. The UK gallon is therefore about 20 percent bigger, and the same car shows a mpg figure about 20 percent higher in the UK. A car rated at 30 mpg in an American review would be about 36 mpg in a British one. When comparing figures from different countries, always check which gallon is meant.

What Counts as Economical?

As a rough guide for a car: under about 4.5 L/100 km is very economical — the territory of efficient hybrids and small diesels. Up to about 6.5 L/100 km is economical, typical of small petrol cars and modern family diesels. Around 6.5 to 9 L/100 km is average for family petrol cars and many SUVs. Above 9 is thirsty, and above 12 is very thirsty, typical of large SUVs, pick-ups and performance cars.

Measuring Your Own Consumption

To measure a trip, fill the tank to the top and reset the trip meter. Drive as usual, then fill the tank again, noting the fuel added and the distance on the trip meter. That fuel is exactly what the car burned. One tank can be distorted by an unusual trip or an early pump cut-off, so averaging several fill-ups gives the most reliable figure.

Consumption for Vans and Motorbikes

The same units apply to any vehicle. Small motorbikes often use 2 to 4 L/100 km, while vans and pick-ups commonly use 9 to 14.

Understanding Your Result

Fuel consumption gives the figure in litres per 100 km.

Kilometres per litre gives the distance per litre.

US mpg gives miles per US gallon.

UK mpg gives miles per imperial gallon.

Rating describes how economical the figure is for a car.

Worth knowing explains the difference between US and UK gallons.

When Should You Use This Calculator?

Reading a car review from another country.

Importing or buying a car rated in unfamiliar units.

Checking your own car's real consumption after a trip.

Moving abroad and comparing running costs.

Comparing cars fairly across units.

Common Mistakes

Treating US and UK mpg as the same.

Multiplying instead of dividing when converting to L/100 km.

Thinking lower mpg is better, or higher L/100 km is better.

Judging savings by mpg steps instead of fuel used.

Measuring from a single tank.