There are two types of viscosity: kinematic viscosity and dynamic viscosity. Kinematic viscosity measures the comparative rate at which a liquid or gas flows. Dynamic viscosity measures a gas's or liquid's resistance to flow as force is applied to it. You must know both the kinematic and dynamic viscosity of a gas or liquid to calculate its density. Knowing just one of the values is not enough, because neither viscosity value has a direct enough mathematical relationship to density.

Write the equation for density, given both the dynamic and kinematic viscosity of a substance. The equation is:

Density = Dynamic viscosity / Kinematic viscosity

Substitute both the values for dynamic and kinematic viscosity into the equation for density. For example, consider a fluid with a dynamic viscosity of 6 Pascal seconds and a kinematic viscosity of 2 square meters per second, the equation would look like this:

Density = 6 / 2

Perform the calculation and express the density in kilograms per cubic meter. In the example, the answer would look like this:

Density = 6 / 2 = 3 kilograms per cubic meter