How to Calculate Specific Weight
Specific weight, or unit weight, describes the weight of a material per unit of volume. Engineers and scientists refer to this material property for applications such as fluid mechanics, construction and soil science. Often, the specific weight of water, 62.4 pounds per cubic foot, is used as a basis of comparison among materials. For example, the specific gravity of a material is the ratio of its specific weight to the specific weight of water. You can determine the specific weight of a material experimentally or from known material properties.
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Instructions
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Weigh the object for which you want to find the specific weight by using the scale. Record the weight in pounds.
For a liquid, first weigh an empty container and record its weight, then add the liquid to the container. Weigh the container with the liquid in it and subtract the weight of the empty container to obtain the weight of the liquid.
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Measure the volume of a liquid or a loose substance such as soil using the volumetric measuring cup.
For a solid object, add enough water to the measuring cup to completely submerse the object and first record the volume of the water. Drop the object into the measuring cup and note the new volume--subtract the original volume of the water from the volume of the water with the object to find the volume of the object.
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Convert the volume to cubic feet. To convert from ounces to cubic feet, the unit multiplier is 0.001. For example, 10 oz. * 0.001 ft^3 / oz. = 0.01 cubic feet.
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Divide the weight by the volume to find the specific weight. Make sure to include the units of lb / ft^3.
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Tips & Warnings
If you know the specific gravity of the object, multiply it by 62.4 lb / ft^3, the specific weight of water, to find its specific weight.
If you know the density of the object, multiply by the gravitational acceleration--32.2 ft / s^2--to find the specific weight.
