Density is also called as unit weight of substance. It is represented by symbol called row ( p). Density represents the degree of compactness of material. If the material is of more density, it is more compacted material. Density is defined as the ratio of mass to volume. p = m/v Units = kg/m 3 or lb/ft 3 Conversion: 1 kg/m 3 = lb/ft 3.