Dynamic Viscosity vs. Kinematic Viscosity:What's the Difference?

2025-08-05

Difference between Dynamic Viscosity and Kinematic Viscosity

Viscosity is the most important property of a lubricant. It's defined as a fluid's resistance to flow and shear. This resistance is measured in two different ways, which can sometimes be confusing. This article explains the difference between these two methods.

Dynamic Viscosity

Definition: A measure of the "friction" within a liquid.

Unit: commonly used mPa·s or cP.

Function: Reflects the flow resistance of lubricating oil under shear force and is often used for analysis of high shear scenarios.

Kinematic Viscosity

Definition: Dynamic viscosity divided by the density of the liquid, which indicates the fluidity of the liquid under the action of gravity.

Unit: usually mm²/s or cSt.

Function: It is widely used in the classification of lubricants (such as ISO VG, etc.) and is an important reference indicator for the selection of lubricants.

The relationship between them :

Kinematic viscosity ( mm2/s ) = dynamic viscosity ( Pa·s ) / density ( g/cm³ )

This means that if the dynamic viscosity and density of a lubricant are known, its kinematic viscosity can be calculated, and vice versa.

Differences in application scenarios in the industry:

Application Scenario	More attention to dynamic viscosity	Speed Range
High-precision machinery, lubricating film analysis	✔
Lubricant classification (such as ISO / VG)		✔
Evaluation of shear performance at high temperature	✔
Flowability and pumpability at room temperature		✔

Conclusion

Simply put, dynamic viscosity focuses on the "friction" within the lubricant, while kinematic viscosity takes into account the oil's density and reflects its actual fluidity in use. When selecting lubricants, understanding these two indicators can help you more accurately judge the oil's performance and ensure the smooth operation of your equipment.