A gallon of water at room temperature weighs 8.33 pounds.

Weight of Water per Gallon 

The most widely accepted simple scientific answer to this question is that a gallon of room temperature water weighs 8.33 pounds. What's more, there is an approximate difference of half a pound per cubic foot of any quantity of water when it is frozen at 32 degrees Fahrenheit vs. when it is at the point of boiling at 100 degrees Fahrenheit.

Molecular Weight of Water 

The weight of water can also be quantified by its molecular weight, the sum of the atomic weights of the atoms in a single molecule. The scientific elements table abbreviation for water - H2O - refers to the fact that a single molecule is made up of two molecules of hydrogen and and one molecule of oxygen.  Given that a molecule of hydrogen weighs one Atomic Mass Unit (AMU) and one molecule of oxygen weighs 16 AMUs, the molecular weight of water is 18 AMUs.

Quick Facts Water Chart

• Weight: 8.33 pounds/gallon  
• Weight: 62.416 pounds per cubic foot at 32^°F
• Weight: 61.998 pounds per cubic foot at 100^°F
• Weight: 0.036 pounds/cubic inch
• Density: 1 gram per cubic centimeter (cc) at 39.2^°F
• Density: 0.95865 gram per cc at 212^°F

                                 Source: US Department of Interior Geological Survey

Notes of Water Interest 

Although waterbeds go in and out of vogue, there is no doubt that they represent one of the most common tactile household experiences of water weight. While a waterbed weighs less per square foot than a refrigerator, the amount of water contained in a king-size waterbed can produce a total weight of approximately 1,500 pounds.  Anyone who has ever tried to move a fully filled waterbed without the proper assistance can attest to how cumbersome these items can be.

Finally, the relative density of other physical objects is often filtered through the prism of whether or not they can sink or float in water. There is even a popular reoccurring feature on The Late Show, "Will It Float?", wherein David Letterman and sidekick Paul Schaeffer try to guess whether or not a particular item will stay on top of the water surface or drop like a stone to the bottom. As a point of interest, salt water has a density that is 3% higher than fresh water.