The weight of a U.S. gallon of milk will vary slightly depending on the liquid's density, but it will normally be around 8.6 pounds. Changes in the fat content of the milk will make the gallon weigh slightly more or less.
The U.S. gallon is equivalent to around 3.79 liters. There are also other units called "gallons." The imperial gallon, which is used in the United Kingdom, Canada and some Caribbean countries, is around 4.546 liters. The U.S. dry gallon, equivalent to about 4.4 liters, is rarely used. The imperial and U.S. dry gallons will weigh more than the standard U.S. gallon.