Newton's law of viscosity states that the shearing stress and the rate of deformation are directly proportional through a constant of proportionality, which is called viscosity, τ = μ du/dy, where
The dynamic viscosity units are N·s/m², Pa·s or kg/m·s, where
In the metric centimeter-gram-second system, the dynamic viscosity units can also be expressed as g/cm·s, dyne.s/cm² or poise (p) where
In the industry of petroleum, centiPoise (cP) or milliPascal-second (mPa·s) is offen used as the unit of dynamic viscosity.
The ratio of dynamic viscosity to density of the fluid is the kinematic voscosity, ν = μ / ρ, where
SI uses Stokes (St) as the unit of kinematic viscosity. 1 Stokes (St) is the kinematic viscosity of a fluid with dynamic viscosity of one P and volumic mass 1 g/cm³.
Another frequent used unit of kinematic viscosity is Centistokes (cSt). 1 Centistokes (cSt) is the kinematic viscosity of a fluid with dynamic viscosity of 1 cP and volumic mass 1 g/cm³.
The efflux time in seconds of 60 ml of fluid flowing though a calibrated Universal orifice under the coditions prescribed by test method ASTM D 88, is the Saybolt Universal Viscosity reported as Saybolt Universal Seconds (SUS) at a specified temperature.