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A gallon is a measure of volume primarily used in the United States but also commonly used in many other English speaking countries. Most other countries use "liters" - the metric unit of measure for volume.
The US gallon is used in the United States and is equal to exactly 231 cubic inches or 3.785411784 liters.
The Imperial gallon or UK gallon is used in the United Kingdom and is equal to approximately 277.42 cubic inches. Its exact value is defined as 4.54609 liters. One imperial gallon is approximately 1.2 US gallons.