The density of a material is defined as its mass per unit volume. The symbol of density is ρ (the Greek letter rho). In some countries (for instance, in the United States), density is also defined as its weight per unit volume. (in wikipedia)

Mathematically: Density = Mass Divided By Volume

where: is the density, is the mass, is the volume.

Different materials usually have different densities, so density is an important concept regarding buoyancy, metal purity and packaging. In some cases density is expressed as the dimensionless quantities specific gravity (SG) or relative density (RD), in which case it is expressed in multiples of the density of some other standard material, usually water or air/gas.

For a homogeneous object, the mass divided by the volume gives the density. The mass is normally measured with an appropriate scale or balance; the volume may be measured directly (from the geometry of the object) or by the displacement of a fluid. Hydrostatic weighing is a method that combines these two. If the body is not homogeneous, then the density is a function of the position: , where is an elementary volume at position . The mass of the body then can be expressed as

The density of a solid material can be ambiguous, depending on exactly how its volume is defined, and this may cause confusion in measurement. A common example is sand: if it is gently poured into a container, the density will be low; if the same sand is compacted into the same container, it will occupy less volume and consequently exhibit a greater density. This is because sand, like all powders and granular solids, contains a lot of air space in between individual grains. The density of the material including the air spaces is the bulk density, which differs significantly from the density of an individual grain of sand with no air included.