1 gallon of water weighs in at around 8.34 lbs which makes water a bit heavier than milk per gallon.
Milk for some reason weighs in a bit less than a gallon of water.
Also the weight of a gallon of water or any amount of water can fluctuate depending on the temperature of the outside.
So if you have 2 gallons of water the 2 gallons of water would weigh almost 17 lbs depending on the temperature.