Density is the mass per unit volume of a material, while specific gravity is the density of a material divided by the density of water. Multiplying the specific gravity by the density of water gives the density of a material.
The usage of specific gravity notation is useful in the metric system, where the density of water is typically taken as 1000 kilograms per meter cubed. Multiplying the specific gravity of a material by 1000 yields the density of the material in kilograms per meter cubed. Dividing 1000 by itself yields the specific gravity of water, which is one.
The larger the specific density of a material, the more dense the material is than water. Specific density can be used to determine whether solids of different materials float or sink in water. If the material specific gravity is greater than one, it sinks in water. If it is smaller than one, it floats in water. Because the density of water can vary at different temperatures, the water density used to determine the specific gravity is often defined at the temperature at which water is densest, 4 C. Specific gravity can also be used to determined concentrations in industrial processes because solutions are almost always less or more dense than pure water.