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.
In an effort to provide healthier substitutes for traditional whole milk, some milk producers skim a portion of the fatty content before packaging it as low-fat or 2 percent milk. This removed portion is sometimes used to make milk by-products like cheese and cream. Because of the missing fat, and the fact that milk is approximately 87 percent water, skim milk generally has a thinner consistency and a more watery taste.