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3.1: It's All About the Weight

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    Water is essential for the survival of all life forms. It makes up approximately 60% of our total body weight and as much as 75% of the earth’s surface. So, how much does water actually weigh? There are a few variables, such as temperature, that determine the weight of water, but for all practical purposes in waterworks mathematics, water weighs 8.34 pounds per gallon. The density (mass per unit volume) of water is 1.00. This is also referred to as specific gravity. When discussing specific gravity, many things are compared to water. For example, if something has a specific gravity less than water (<1), then the substance will float on water. Conversely, if a substance has a specific gravity greater than 1, it will sink in water.

    The table below lists common specific gravities/densities and weight of substances used in the waterworks industry. Remember, these are only examples and should not be put to memory. On any State exam, you will be given the specific gravity or corresponding weight of the substance in the question.


    Specific Gravity


    Crude Oil


    6.80 lbs/gal



    8.34 lbs/gal

    Chlorine (g)


    20.77 lbs/gal

    Calcium hypochlorite


    19.60 lbs/gal


    1.16 – 1.40

    9.67 – 11.68 lbs/gal

    Ferric chloride


    11.93 lbs/gal


    Since water is the reference, then a specific gravity (SG) of 1 and a weight of 8.34 lbs/gal are the numbers needed to calculate the SG and weight of other substances.

    What is the weight of a substance in lbs/gal if it has a SG of 1.25? Remember, anything that has a SG >1 will weigh more than 8.34 lbs/gal.

    • 8.34 lbs/gal1 SG x 1.25 SG1 = 10.43 lbs/gal

    What is the SG of a substance that weighs 5.75 lbs/gal? Remember, anything with a weight <8.34 lbs/gal will have a SG <1.

    • 1 SG8.34 lbs/gal x 5.75 lbs/gal1 = 0.69 SG


    Solve the following density related problems.

    3.1: It's All About the Weight is shared under a CC BY license and was authored, remixed, and/or curated by Mike Alvord.

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