This article uses both gallon and liter as a unit of measurement for water. You will likely be most familiar with liter, as most of the world uses this unit of measurement for liquids. In the U.S. and the U.K., however, gallon is still commonly used. However, the size of one gallon differs in these two countries. In the U.K., the imperial gallon is about 4.5 liters. The U.S. gallon is about 3.8 liters. When gallon is used in this unit, it refers to the U.S. gallon.