Viscosity is inversely proportional to density. This relation applies to fluids with viscosity referring to kinematic viscosity and not dynamic viscosity.
A substance's density refers to the amount of matter that exists in its unit volume. Density can be of mass density, specific weight or relative density. Mass density, which is symbolized by the Greek letter, rho, is equivalent to a substance's mass divided by its volume. Its unit is kilograms per cubic meter or kg/m3. For example, water's density is taken to be 1000 kg/m3 at room temperature and normal atmospheric pressure. A fluid's viscosity refers to its resistance to continuous movement or sheer deformation. Kinematic viscosity is characterized by the ratio of dynamic viscosity (symbolized by the Greek letter, mu) to mass density.