The density of a material is defined by the equation: density = mass / volume, ρ = m / V, where density, ρ, in kilograms per metre cubed, kg/m³ mass, m, in kilograms, kg, volume, V, in metres cubed, m³.

There 3 main states of matter: solids (defined shape, regularly arranged particles, high density), liquids (can change shape, particles can move, lower density), gases (can change shape, randomly distributed particles, low density).