Density is defined as mass per volume, and is most often expressed in g/cm³ (=g/mL).

For reference, the density of air at sea level is ~1.2 kg/m³, the density of water is 1000 kg/m³ (1 g/cm³), and the density of glass is 2.65 g/ cm³, and of gold is 19.3 g/cm³.