Quantification of the mass and volume of materials is essential for the purpose of scientific inquiry, as well as business. To use any material in industrial production, it's necessary that its mass, volume, and other physical quantities are measured. While purchasing any food item or other supplies, you pay according to an object's weight.
Gallon is the most common unit, used to specify liquid volume in USA and UK. Right from gasoline to cans of water, you find the volumes of most liquids quoted in liters. Here, we deal with the US liquid gallon. To summarize, a gallon of water weighs 3785.41178 gm.
Different Gallon Units
For uniformity of usage, all the standard units of measurement are defined by national or international organizations, to eliminate confusion. Of the three definitions of gallon, the one that is most relevant to this discussion is the US liquid gallon.
One US liquid gallon is defined to be 231 cubic inches, which is 3.785411784 liters. Since 1 liter is 1000 cubic centimeters, a US liquid gallon is 3785.411784 cc. This is equal to 0.133680555 cubic feet. From the knowledge of the density and volume of a liquid, its mass can be calculated. The relation that you need to use is 'Density = Mass / Volume'.
Weight in Grams
A gallon of any liquid is exactly 3785.411784 cc. Water's density in a normal temperature range is approximately 1 gm/cc. Therefore, the weight of one gallon of water is simply (3785.411784 cc) x 1gm/cc = 3785.41178 gm. That is, it weighs about 3.78541178 kilograms.
Weight in Pounds
One kilogram equals 2.20462262 pounds and one gallon of water is 3.78541178 kg. Therefore, one gallon weighs (3.78541178 x 2.20462262) lbs, which is equal to 8.3454 lb.
Weight in Ounces
You might need to know the exact weight of water in ounces. One gallon weighs about 133.526471 Oz.
How Much Does a Gallon of Salt Water Weigh?
The weight of a gallon of salt water will vary according to the amount of salt dissolved. The total mass is the mass of water, plus the mass of salt dissolved in it. So salt density and the nature of the dissolved salt needs to be known to know the weight of salt water. It is best to weigh the salt using a standard digital balance, to get an accurate value.
Such unit conversions between gallons and pounds or gallons and kilograms are best put in a chart for easy reference, if you need this information on a regular basis.