A gallon is a unit of measurement for liquids, equal to four quarts or about 3.8 liters. In the United States, when you buy a gallon of milk at the store, you're getting a large jug that contains 128 U.S. fluid ounces. A typical bathtub holds about 40 gallons of water, while a car's gas tank might hold 12 to 15 gallons.

The gallon helps us measure and talk about larger amounts of liquid in everyday life. Instead of saying “I need 128 ounces of paint,” you can simply say “I need a gallon.” Swimming pools are measured in thousands of gallons, and city water towers might hold a million gallons or more.

Americans use gallons constantly, but most other countries use liters instead, which can cause confusion when traveling abroad. If you're trying to figure out how much liquid something holds, remember that a gallon is roughly the size of a large milk jug in the United States.