Density is a measure of the quantity of mass in a unit volume of material. Unit weight is a measure of the weight of a unit volume of material. Both can be used interchangeably. The units of density are ton/m³, kg/m³ or g/cm³. The following are the basic weight relations:
1. The ratio of the mass of water present to the mass of solid particles is called the water content (w), or sometimes the moisture content.
Its value is 0% for dry soil and its magnitude can exceed 100%.
2. The mass of solid particles is usually expressed in terms of their particle unit weight 
or specific gravity (Gs) of the soil grain solids .
where
= Unit weight of water
For most inorganic soils, the value of Gs lies between 2.60 and 2.80. The presence of organic material reduces the value of Gs.
3. Dry unit weight 
is a measure of the amount of solid particles per unit volume.
4. Bulk unit weight 
is a measure of the amount of solid particles plus water per unit volume.
5. Saturated unit weight
is equal to the bulk density when the total voids is filled up with water.
6. Buoyant unit weight
or submerged unit weight is the effective mass per unit volume when the soil is submerged below standing water or below the ground water table.
