How much does a gallon of fat free milk weigh?

The weight of a gallon of milk is only slightly greater than a gallon of water, about 3% more. A US gallon with 2% milkfat weighs about 8.4 pounds, compared to 8.6 pounds for "whole milk" and 8.35 pounds for a gallon of water.

Milk is 87 % water, thus a gallon of milk will not differ from milk much.

The 8.6 lbs/gal factor (whole milk) is correct. The weight increases as fat is removed.

By weight:

Milk : 8.6 lb/gallon

Condensed milk : 9.4 lb/gallon

Cream 8.28 lb/gallon

* The British or Imperial Gallon is defined as exactly 10 pounds of water. You can convert the above figures by using 1 Imperial Gallon = 1.201 US gallons.