If you mean to convert miles per gallon into litres per kilometre then these are the figures you need.
1 Imperial Gallon (UK) = 4.54609188 litres
1 mile = 1.609344 kilometres
Multiply your gallons used by 4.54 and divide that into the miles you have done and you will have miles per litre.
Multiply the result by 1.6 and you will have kilometres per litre.