(1) Bulk mass density or Bulk mass density or Simply mass density or Density :- The bulk mass density $(\rho)$ is defined as the total mass (M) per unit total volume (V)

$\rho = \dfrac{M}{V}$

Units:- Kg/$m^3$ ,gm/ml, mg/$m^3$

(2) Dry mass density:- Mass of solids per unit volume.

$\rho_d = \dfrac{M_s}{V}$

(3) Standard mass density:- The standard mass density is defined as bulk mass density of the soil when it is fully saturated.

$\rho_{sat} = \dfrac{M_{sat}}{V}$

(4) Submerged mass density:- Submerged mass density$(\rho^!)$ of the soil is defined as the submerged mass per unit of the total Volume.

$\rho^! = \dfrac{M_{sub}}{V}$

NOTE:- The Submerged density is also expressed as $\rho_{sub}$ in some texts. It is also known as Buyount mass density $(\rho_b)$

(5) Mass density of Solids:- The mass density of the solids $\rho_s$ is equal to ratio of mass of solids to the volume of solids.

$\boxed{\rho_s = \dfrac{M_s}{V_s}}$