The term density is used herein to denote the mass-to-volume ratio of a material. However, some
references, particularly older ones, use the term to describe unit weight. Density is denoted by p. Because
m = W/g, the unit weight terms defined above can be converted to mass densities as follows:
In the SI system mass densities are commonly expressed in Mg/m3, kg/m3, or g/ml. The mass density of water can therefore be expressed Pw = 1000 kg/m3 = 1 Mg/m3 = 1 g/ml. The mass density of soil solids typically ranges from 2640 to 2750 kg/m3. Where mass or mass density values (g, kg, or kg/m3) are given or measured, they must be multiplied by g(9.81 m/s2) to obtain weights or unit weights before performing stress calculations. In the English system mass density values are virtually never used in geotechnical engineering and all work is performed in terms of unit weights (lb/ft3).