Understand the relationship for specific gravity and the common units involved. Specific gravity is the density of a material divided by the density of water the the reference point of 39 degrees Fahrenheit (4 degrees Celsius). The most common unit of measurement for specific gravity is grams per cubic centimeter (g/cm³). This is because the density of water is 1 g/cm³, and it makes comparing the specific gravity of gases much simpler.
Find the density of a gas through a common reference or calculation. If calculating density from observations (such as when performing a laboratory experiment), recall that density is equal to the mass of the gas divided by the volume it occupies. If looking up a gas's density in a reference, remember that temperature and pressure will affect the density of the gas.
Convert the density to the same units as the density used for water at the reference point. If possible, convert to units of g/cm³ so the resulting answer for specific gravity is the exact same. Remember that specific gravity itself doesn't use a unit of measurement; it represents only a ratio between the density of the gas or material and water.