While the mass and volume of a substance from sample to sample of the material, the ratio of the mass to the density remains the same under similar pressure and temperature. It is often useful to compare the density of a substance with the density of a reference material. This comparison is known as the specific gravity. For solids and liquids, water is typically used as the reference density, while dry air at 32 degrees Fahrenheit and one atmosphere of pressure is usually used as the reference density for gases. Since specific gravity is simply a ratio between the densities of the substance, it may be solved by simply dividing between the density of the substance by the density of the reference material.

### Specific Gravity of Liquids and Solids

Measure the density of the liquid or solid substance for which specific gravity will be calculated. Density may be calculated by dividing the mass of the substance by the volume (density = mass / volume). While mass is usually determined using a balance, a variety of techniques are employed for measuring the volume depending on the substance, such as graduated cylinders for liquids, rulers for measuring the sides of a regular solid, or water displacement for insoluble irregular solids.

Reference the density of the reference material, in this case the fresh water at 39 degrees Fahrenheit. At this temperature fresh water is at its greatest density at 1 gram per milliliter.

Divide the density of the original substance by the density of the reference material to yield the specific gravity. For example, the density of gold is 19.3 grams per milliliter. This gives gold a specific gravity of 19.3. In other words, gold is 19.3 times denser than fresh water at 39 degrees Fahrenheit. Note that using water at 39 degrees Fahrenheit allows the specific gravity to be the same as the density, though specific gravity is dimensionless and lacks units of measure.

### Specific Gravity of Gases

Measure the density of the gas for which specific gravity is to be calculated. Again, the density is the ratio between the mass and volume of the substance. It is also extremely important to note the temperature and pressure of the gas when these measurements are recorded, since the volume of a gas, and consequently the density, varies greatly with changes in pressure and temperature.

Reference the density of dry air at a temperature of 32 degrees Fahrenheit and a pressure of one atmosphere. Dry air is air in which the moisture content is removed. The density of dry air at this temperature and pressure is 1.2929 grams per cubic meter. It should be noted that sometimes a different temperature for the dry air reference is used, and consequently, the dry air has a different density than at 32 degrees Fahrenheit and one atmosphere. The temperature and pressure should be recorded when presenting the final specific gravity. Also ensure the density for both the gas and the dry air are given in the same units of measure. While metric units are the standard, densities are often given in grams per liter as well as grams per cubic meter. Additionally, densities may also be given using avoirdupois measurements such as pounds per cubic foot. If the density units differ conversion to the desired units may be required.

Divide the density of the substance by the reference density for dry air to yield the specific gravity of the substance. Again, specific gravity is a dimensionless quality with no units of measure. For example, the density of carbon dioxide at 32 degrees Fahrenheit and one atmosphere is 1.9769 grams per cubic meter. Dividing the density of carbon dioxide by the density of dry air gives a specific gravity of 1.529.