Viscosity & Rheology Classroom

The viscosity of a fluid is a measure of its resistance to gradual deformation by shear stress or tensile stress. It is a measure of the resistance of flow due to internal friction when one layer of fluid is caused to move in relationship to another layer. 

Viscosity is a property of the fluid which opposes the relative motion between the two surfaces of the fluid that are moving at different velocities. In simple terms, viscosity means friction between the molecules of fluid. When the fluid is forced through a tube, the particles which compose the fluid generally move more quickly near the tube's axis and more slowly near its walls; therefore some stress (such as a pressure difference between the two ends of the tube) is needed to overcome the friction between particle layers to keep the fluid moving. For a given velocity pattern, the stress required is proportional to the fluid's viscosity.


Video courtesy of Ekeeda - 

Absolute Viscosity: The tangential force per unit area of two parallel planes at unit distance apart when the space between them is filled with a fluid and one plane moves with unit velocity in its own plane relative to the other. Also known as coefficient of viscosity.

Apparent Viscosity: The value obtained by applying the instrumental equations used in obtaining the viscosity of a Newtonian fluid to viscometer measurements of a non-Newtonian fluid.

Dilute Solution Viscosity: The viscosity of a dilute solution of a polymer, measured under prescribed conditions, is an indication of the molecular weight of the polymer and can be used to calculate the degree of polymerization.

Kinematic Viscosity: The absolute viscosity of a fluid divided by the density of the fluid. Also known as the coefficient of kinematic viscosity. Measured in stokes (St) or centistokes (cSt). 

Intrinsic Viscosity ([η]): The ratio of a solution’s specific viscosity to the concentration of the solute, extrapolated to zero concentration. Intrinsic viscosity reflects the capability of a polymer in solution to enhance the viscosity of the solution. 

General Information

Articles & White Papers

Our scientists contribute to industry knowledge by authoring papers in various technical publications. See below for articles and white papers pertaining to:

In additional to technical articles and white papers, we offer a variety of short Application Proofs as examples of our testing capabilities. Our application scientists are available to discuss your particular application needs and help you find solutions to your most pressing testing issues.


Basic Viscosity
Shear Stress / Shear Rate = Viscosity

a = 9.801 m/s²

Dynamic Viscosity
1 poise = 1 g/cm·s = 0.1 N·s/m² = 0.1 Pa·s
1 cP = 1 mPa·s

SI Unit Conversion
1 Pascal = 1 Newton / m²

Kinematic Viscosity
1 Stoke = 1 cm²/s = 100 mm²/s
1 cSt = 1 mm²/s

Conversion from Rotational to Kinematic Viscosity
Poise / Density = Stoke
g/cm·s / g/cm3 = cm²/s

Conversion from Kinematic to Rotational Viscosity
Stoke * Density = Poise
cm²/s * g/cm3 = g/cm·s

Conversion from Kinematic to Saybolt Viscosity

Krebs Unit (for Thomas®-Stormer Viscometer)
Over the viscosity range of 200 to 2100 mPa·s (cP)
ln(KU) = 1.1187 + 0.8542*ln(0.1938v + 36) - 0.0443(ln(0.1938v +36))²

Over the viscosity range of 2100 to 5000 mPa·s (cP)
ln(KU) = 1.8118 + 0.596*ln(0.1938v + 36) - 0.0206(ln(0.1938v +36))²

Where KU is the viscosity in Krebs Units at 25°C
and v is the viscosity in mPa·s (cP) at 25°C

Interested in getting more information on how a viscometer can assist with your work? 

Contact Us 

We are using Cookies to improve your user experience. If you want to learn more about cookies and how we use them, please visit our cookie page. Further browsing of this site indicates that you accept our cookie policy.