The weight of 1 gallon of pure water at room temperature is 8.329 pounds. The weight varies with temperature, ranging from 8.344 pounds at 32 F, the freezing point, to 7.996 pounds at 212 F, the boiling point. Water is at its heaviest at 39.2 F.
Water's weight varies with how densely its molecules pack together. At high temperatures, the molecules are farther apart. Impurities, including minerals, gases, bacteria or dissolved chemicals, affect the weight of water from common sources such as the tap or a pond. Natural seawater, with a roughly 3.5 percent salt concentration, weighs about 8.56 pounds per gallon. Ice, the solid form of water, is 9 to 10 percent less dense than liquid water, which is why it floats.