Learn how to calculate the weight of a plastic object when a scale is not available. The weight of an object depends on its weight density as well as its volume. (See References 1) Weight density measures the compactness of a material. For example, polypropylene has a weight density of 0.032 lb. per cubic inch. (See References 2) Volume is a direct measure of the space an object occupies. Normally, volume is calculated for physical dimensions, but for an irregularly shaped object this does not work. Archimedes' principle states that when an object is submerged in water the amount the liquid rises equals the object's volume. (See References 3)
Fill a graduated beaker or other measuring container with water. Place enough water in the container so that when the plastic object is submerged it is completely covered. Also, be sure the container will not overflow. For example, the initial water level might be 120 fluid ounces.
Place the plastic object into the water. Record the new water level on the side of the container. For example, the final water level could be 150 fluid ounces.
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Subtract the initial volume from final volume to get the volume of the plastic object in fluid ounces. Performing this step, for the example, leads to 150 fluid ounces minus 120 fluid ounces, or a volume of 30 fluid ounces.
Convert the volume to cubic inches by multiplying by 1.80, because a single fluid ounce equals 1.80 cubic inches. (See References 4) This step, for example, yields 30 fluid ounces times 1.80 cubic inches per fluid ounce, or a volume of 54.0 cubic inches.
Multiply the volume by the weight density, in lb. per cubic inch, of the specific plastic in use to arrive at the weight of the object in pounds. (See References 1) Consult a reference table of plastic properties if unsure of the weight density. For example, assume the object is made of polypropylene. Completing the sample exercise leads to 54.0 cubic inches times 0.032 lb. per cubic inch, or a weight of 1.7 lb.