Density and specific gravity have very similar, but not quite identical definitions. Density is the amount of something per unit volume. Most typically, one expresses the mass per unit volume for a solid or liquid. For example, 5.2 g/cm3. For gases or dusts we might express this as g/m3. Specific gravity is a ratio of the mass of a material to the mass of an equal volume of water at 4 oC (39 oF). Because specific gravity is a ratio, it is a unitless quantity. For example, the specific gravity of water at 4 oC is 1.0 while its density is 1.0 g/cm3. Relative density is essentially the same as specific gravity, however the temperature used for the water (or even another material) is not necessarily 4 oC. For this reason, a relative density measurement will include the temperatures used for both materials. For example, "relative density 15/0: 0.87" indicates that the density of the material was determined at 15 oC and it is being divided by the density of water at 0 oC. The temperatures may also be indicated as a superscript (material) and subscript (water) after the numeric value.