What is the difference between dynamic and kinematic viscosity?

Dynamic viscosity (μ) measures a fluid's resistance to shear flow, essentially its 'stickiness'. Kinematic viscosity (ν), on the other hand, describes a fluid's resistance to flow under the influence of gravity, taking into account both its dynamic viscosity and its density. The relationship is ν = μ / ρ.

What are the common units for dynamic and kinematic viscosity?

Common units for dynamic viscosity include Pascal-seconds (Pa·s) in SI, and centipoise (cP) or millipascal-seconds (mPa·s) (where 1 Pa·s = 1000 cP). For kinematic viscosity, common units are square meters per second (m²/s) in SI, and centistokes (cSt) or square millimeters per second (mm²/s) (where 1 m²/s = 1,000,000 cSt).

Why is fluid density important for kinematic viscosity?

Kinematic viscosity accounts for how a fluid flows under its own weight. This flow is affected not only by the internal friction (dynamic viscosity) but also by the fluid's inertia, which is directly related to its density. A denser fluid, even with the same dynamic viscosity, will have a lower kinematic viscosity because its greater mass helps it overcome internal friction more easily under gravity.

Dynamic and kinematic viscosity

Definition and conversion dynamic to kinematic viscosity

Follow us on Twitter
Question, remark ? Contact us at contact@myengineeringtools.com

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.

⚠️ ENGINEERING NOTICE & EDUCATIONAL DISCLAIMER: This interactive calculator is provided exclusively for preliminary estimation and educational purposes. It is not intended for detailed design or equipment procurement without certified vendor rating. No warranty, expressed or implied, is provided, and no liability is assumed.

Dynamic Viscosity (μ)

Fluid Density (ρ)

Results will appear here.

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 :

\\[ \\nu = \\frac{\\mu}{\\rho} \\]

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

⚙️ Practical Engineering Considerations for Viscosity

Temperature Dependency: Viscosity is highly dependent on temperature. For most liquids, viscosity decreases significantly with increasing temperature. Always specify the temperature when reporting viscosity.
Pressure Dependency: While less pronounced than temperature, liquid viscosity generally increases with increasing pressure. This effect is more significant for gases.
Non-Newtonian Fluids: Many industrial fluids (e.g., slurries, polymers, certain food products) are non-Newtonian, meaning their viscosity changes with shear rate. This calculator assumes Newtonian behavior (constant viscosity). For non-Newtonian fluids, advanced rheological analysis is required.
Measurement Accuracy: Obtaining accurate viscosity and density data is crucial for process design. Rely on laboratory measurements or certified vendor data for critical applications. Online correlations can provide estimates but should be verified.
Fluid Flow Regimes: Viscosity is critical in determining flow regimes (laminar vs. turbulent) via the Reynolds number, which impacts pressure drop, heat transfer, and mixing.
Pumping & Transport: High viscosity fluids require more pumping power and often larger pipe diameters to maintain acceptable flow rates and pressure drops. Ensure pump selection accounts for fluid viscosity at operating conditions.