Gallon (gal)

The gallon is the big unit of liquid volume in the United States, used for milk jugs, paint, water tanks and gasoline. It divides into 4 quarts or 8 pints.

Its size dates from colonial times. American colonists adopted the English wine gallon, while England also used a larger ale gallon (NIST Handbook 44, Appendix B). In 1824 Britain replaced both with the imperial gallon, based on the space filled by 10 pounds of water, but the US kept the wine gallon, so the two gallons still differ.

In US grocery stores, milk, juice and distilled water usually come in gallon and half-gallon jugs. A half gallon holds 2 quarts, or 64 fluid ounces.

Gasoline is priced per gallon, but the pump does not count in quarts or pints. NIST notes that most price-computing pumps show tenths, hundredths or thousandths of a gallon instead (NIST Handbook 44, Appendix B).

An imperial gallon equals 1.201 gallons in US measure, so it is about a fifth larger. Both gallons split into 4 quarts and 8 pints, which means every British quart and pint is larger by the same fifth.

Only the fluid ounce breaks the pattern, because the two countries divide their gallons into different numbers of fluid ounces. If a British recipe or an older Canadian one mentions gallons, it means the imperial gallon.