How much does water weigh per gallon?
Water weighs 8.34 pounds per gallon. Here, the density of the water which can be measured in pounds is affected by temperature and pressure. The site http://www.engineeringtoolbox.com/water-density-specific-weight-d_595.html should give you the different specifications.
there are 128 ounces in a gallon and each ounce weights 1/16th of a pound. 128X1/16 equals 8 pounds, the weight of a gallon of water.