The quart is a unit of volume equal to a quarter of a gallon. Three kinds of quarts are currently used: the liquid quart and dry quart of the US customary system and the imperial quart of the British imperial system. All are roughly equal to one liter. It is divided into two pints or four cups. Historically, the exact size of the quart has varied with the different values of gallons over time and in reference to different commodities.
One-quart milk jugs (top shelf); half-gallon (two-quart) milk jugs (bottom shelf)
The gallon is a unit of volume in British imperial units and United States customary units. Three different versions are in current use:the imperial gallon, defined as 4.54609 litres, which is or was used in the United Kingdom, Ireland, Canada, Australia, New Zealand, and some Caribbean countries;
the US gallon, defined as 231 cubic inches, which is used in the United States and some Latin American and Caribbean countries; and
the US dry gallon ("usdrygal"), defined as 1⁄8 US bushel.
A one-US-gallon gas can showing "U.S. Gallon" marking (for American use), imperial gallons (for British use), and litres (for Canadian use)
A Shell petrol station selling 2* and 4* (leaded petrol) by the gallon in the UK, circa 1980
A fuel station in the United States displaying fuel prices per US gallon
An American milk bottle with a volume of one US gallon
