Mass of a unit volume of a material substance.
It is calculated by dividing an object's mass by its volume. In the International System of Units , and depending on the units of measurement used, density can be expressed in grams per cubic centimetre (g/cm 3 ) or kilograms per cubic metre (kg/m 3 ). The expression "particle density" refers to the number of particles per unit volume, not to the density of a single particle. See also specific gravity .