Centipoise (cP) vs. Centistokes (cSt)

Formulators typically report viscosity values in units of centipoise (cP), which is a unit of dynamic viscosity (η) in the centimeter gram second (cgs) system of units; one cP = 0.01 g·cm-1·s-1. However, the viscosities of dimethicone fluids are reported in units of centistokes (cSt), which are units of kinematic viscosity (μ); one cSt = 0.01 cm2·s-1. A fluid’s dynamic viscosity at a given temperature is simply its kinematic viscosity multiplied by its density (ρ) at that temperature:

η = μ · ρ

For example, a dimethicone fluid with a kinematic viscosity of 100 cSt and a density of 0.964 g·cm-3 at 25°C would have a dynamic viscosity of 96.4 cP. Kinematic viscosity is commonly measured by using a calibrated glass capillary viscometer, whereas measurements of dynamic viscosities require more sophisticated instruments such as rheometers or rotational viscometers, e.g., a Brookfield viscometer.