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What is kinematic viscosity?

The kinematic viscosity [m2/s] is the ratio between the dynamic viscosity [Pa.s = 1 kg/m·s] and the density of a fluid [kg/m3].
The SI unit of the kinematic viscosity is m2/s. Other units are: 1 St (Stoke) = 1 cm2/s = 10−4 m2/s. 1 cSt (centiStoke) = 1 mm2/s = 10−6m2/s.
Water at 20 °C has a kinematic viscosity of about 1 cSt.

One way is to measure a fluid’s resistance to flow when an external force is applied. This is dynamic viscosity.        

The other way is to measure the resistive flow of a fluid under the weight of gravity. The result is kinematic viscosity. Put another way, kinematic viscosity is the measure of a fluid’s inherent resistance to flow when no external force, except gravity, is acting on it.

Measuring Kinematic ViscosityWhat is kinematic viscosity 

There are several ways to find the kinematic viscosity of a fluid, but the most common method is determining the time it takes a fluid to flow through a capillary tube. The time is converted directly to kinematic viscosity using a calibration constant provided for the specific tube.

The unit of measure of kinematic viscosity is Centistokes (cSt).

A basic difference between the dynamic and kinematic viscosity measurements is density. Density actually provides a way to convert between a kinematic and a dynamic viscosity measurement. The formula for the conversion is:

  • Kinematic (cSt) x Density = Dynamic (cP)
  • Dynamic (cP) / Density = Kinematic (cSt)

For a given sample, with a density greater than one, dynamic viscosity will always be the higher number.

Need a Refresher on Density?


Density is the ratio of the mass (or weight) of the sample divided by the volume of the sample. Think about an ice cube and a cube of steel. They may be the same size, but the steel cube weighs more than the ice cube. Therefore, we say that steel has a greater density than ice cube.

The mass (or weight) of a fluid is determined by gravity. In the kinematic measuring method, gravity is the only force that acts on the sample.

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