Fuel consumption in the metric standard is litres per 100 kilometres: L/100km = fuel used (L) ÷ distance (km) × 100. A lower number means a more economical car. The tool also shows the inverse — distance per litre (km/L) = distance ÷ fuel — which is handy when comparing with mileage figures.
If you enter a price per litre, it computes cost per kilometre = (fuel ÷ distance) × price and the total fuel cost = fuel × price, so you can see exactly what a trip costs to drive. Leave the price blank to get consumption only. Everything is calculated locally in your browser; nothing is uploaded.