Density equals:
weight/area
weight/volume
mass/weight
mass/volume

Density can be determined by the mass of an object and how much it takes up space (volume).  Or density is equal to mass over volume. It is represented by the formula D = M/V where D is the density in kg/m^3 or lb/ft^3, M is the mass in kg or lb and V is the volume in m^3 or ft^3. 

For example, you are given the mass of an object of 40.5kg and a volume of 15m^3. Find its density.  

D = M/V
D = (40.5 kg) / (15 m^3)
D = 27/10 or 2.7 kg/m^3