The U.S. customary liquid units have their roots in the 18th century British system. At that time, there were many units of volume, each appropriate to a different commodity, such as water, wine and ale. The United States, which was then Britain's American colonies, accepted the wine gallon as their standard unit of liquid measure. This became today's standard U.S. gallon. The gallon is divided into 4 quarts, 8 pints and 16 cups.
Some units of measure, such as the gill and hogshead, are not widely used except to measure alcohol. A gill is 1/4 of a pint or 1/2 of a cup. A hogshead is 63 gallons. Other units of measure, such as the fluid ounce, tablespoon and teaspoon are widely used in cooking, where they may also be used as units of dry ingredients. One fluid ounce is 1/8 of a cup, 2 tablespoons or 6 teaspoons.
In 1824, Great Britain established a new system of measurement called the imperial system. This system used the same names as the older English and U.S. customary systems, but redefined the size of the units. The imperial gallon is now equal to approximately 1.2 U.S. gallons, and contains proportionally larger quarts, pints, cups and fluid ounces.
Metric, or SI, units are the most common international units of measurement. The metric unit of liquid volume is the liter, which is the volume of a cube that measures 1/10 of a meter on each side. You can create smaller units by adding a prefix that multiplies the liter by a power of 10. For example, the milliliter is 1/1000 of a liter.