There are 3.78541178 liters in a gallon. In calculations, to simply we say 1 gallon is approximately 3.79 liters.
In order to convert gallon to liter, we simply multiply with 3,7854.
For example, if you want to know how many liters 5 gallons milk is, just multiply 5 with 3,7854. It is approximately 18,927 liters.