Specific gravity is the ratio of the density of a substance to the density of water at a given pressure and temperature. Specific gravity is typically measured at 4 degrees Celsius (39.2 degrees Fahrenheit), unless otherwise indicated. According to SIMetric, the density of water at 4 degrees Celsius and standard atmospheric pressure is 62.4 pounds per cubic foot.
Determining specific gravity is a simple two-step process that starts with calculating the density of the substance you want to test. Note that specific gravity is only applicable to liquids and solids, because gases have a variable density.
Measure out 25 milliliters (1.526 cubic inches) of the liquid you intend to test, using the graduated cylinder.
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Place the cup on the scale and measure its weight. Next, add the 25 milliliters of liquid from the graduated cylinder to the cup and weigh the cup again. The difference between the weight of the cup with and without the liquid is the weight of the liquid.
Divide the weight of the liquid by the volume of the liquid to find its density. Density is measured in units of mass per volume (i.e., how many grams or pounds of the substance fit into a specific volume). The density of water, for example, is 1 gram per milliliter, or 62.4 pounds per cubic foot.
Divide the density of the liquid by the density of water to find the specific gravity. In other words, specific gravity equals the density of the liquid divided by 62.4 pounds per cubic foot (if you are using imperial units) or 1 gram per milliliter (if you are using metric).
This calculation provides an approximation of specific gravity, because it does not take temperature into account. However, this approximation will be extremely close at room temperature, and should suffice for all practical intents and purposes.
To measure the specific gravity of a solid, you'll need to determine its volume, which is more complicated than determining the volume of a liquid, and hence beyond the scope of this article. If the solid is a cube or a sphere, measure its dimensions to find the volume. If it is an irregularly shaped object, you may need to use calculus to calculate the volume more precisely.