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Dynamic and kinematic viscosity

Definition and conversion dynamic to kinematic viscosity

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1. Dynamic viscosity
2. Kinematic viscosity

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The viscosity is one of the fundamental properties of fluid used in almost every physical calculations, for example pressure drop calculation. This page is giving the definitions of 2 viscosity, dynamic and kinematic, and how to convert from one viscosity to another.

1. Dynamic viscosity

The dynamic viscosity, often represented by the greek letter μ is a measure of the molecular interactions in the fluid. It is expressed in :

• Pa.s
• mPa.s
• cP (centipoise)

It is possible to convert from unit to another thanks to :

• 1 cP = 1 mPa.s = 0.001 Pa.s

2. Kinematic viscosity

The kinematic viscosity, often represented by the greek letter ν and used mainly for liquids, is a measure of the viscosity in the particular situation where the fluid is flowing down because of its own weight. Such a flow is actually dependent on the dynamic viscosity of the fluid but also its weight, represented by the density.

The conversion from dynamic viscosity to kinematic viscosity can thus be done thanks to the following formula :

ν = μ / ρ

With :

ν = kinematic viscosity (m²/s)
μ = dynamic viscosity (Pa.s = kg/m.s)
ρ = fluid density (kg/m3)

The kinematic viscosity is measured in

• St (Stokes)
• cSt
• m²/s
• mm²/s

The relation in between each unit is :

• 1 St = 10^-4 m²/s
• 1 cSt = 0.01 St
• 1 cSt = 1 mm²/s

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