Dynamic, Absolute and Kinematic Viscosity

Overview

Viscosity quantifies a fluid's internal resistance to flow. When adjacent layers of fluid move at different velocities, inter-molecular friction generates shear stress. Two related measures capture this behavior:

• Dynamic (absolute) viscosity ($\mu$) — the tangential force per unit area needed to move one horizontal plane relative to another at unit velocity while maintaining unit separation.
• Kinematic viscosity ($\nu$) — the ratio of dynamic viscosity to fluid density, expressed without force units.

Both quantities depend strongly on temperature; any viscosity value is meaningless without a reference temperature.

Key Formulas

Newton's Law of Friction

For a Newtonian fluid in laminar flow, the shear stress between fluid layers is proportional to the velocity gradient:

$\tau = \mu \frac{du}{dy}$

where $\tau$ is shear stress (Pa), $du/dy$ is the velocity gradient perpendicular to flow (1/s), and $\mu$ is dynamic viscosity (Pa·s).

Rearranging gives the definition of dynamic viscosity:

$\mu = \frac{\tau}{du/dy}$

Kinematic Viscosity

Kinematic viscosity is obtained by dividing dynamic viscosity by mass density:

$\nu = \frac{\mu}{\rho}$

Other Viscosity Units

Dynamic viscosity is commonly expressed as Pa s, N s/m², poise, centipoise, lbm/(ft s), or lbf s/ft². Kinematic viscosity is commonly expressed as m²/s, Stokes, centiStokes, ft²/s, in²/s, or Saybolt Universal Seconds for petroleum products.

Newtonian Fluids

Newtonian fluids have a constant viscosity at a given temperature and pressure. The shear stress is directly proportional to the velocity gradient.

Shear-thinning or Pseudo-plastic Fluids

Shear-thinning fluids decrease in apparent viscosity as shear rate increases.

Thixotropic Fluids

Thixotropic fluids decrease in viscosity over time when sheared and recover partly or fully when left at rest.

Dilatant Fluids

Dilatant or shear-thickening fluids increase in apparent viscosity as shear rate increases.

Bingham Plastic Fluids

Bingham plastic fluids resist flow until a yield stress is exceeded, then flow approximately like a viscous fluid.

Measuring Viscosity

Dynamic viscosity is commonly measured with rotational viscometers, falling-ball viscometers, and capillary methods. Kinematic viscosity is often measured with calibrated capillary or efflux viscometers such as Saybolt instruments.

Imperial Conversion

When converting from centipoise and specific weight (lb/ft³) to kinematic viscosity in ft²/s:

$\nu = 6.7197 \times 10^{-4} \frac{\mu}{\gamma}$

Variables

Quick Calculator

Default values approximate water at 20 °C — adjust inputs for other fluids.

Example - Air, Convert between Kinematic and Absolute Viscosity

The source example for air is preserved explicitly here. For air with absolute viscosity approximately 1.983e-5 Pa s and density approximately 1.205 kg/m3, the kinematic viscosity is:

$\nu = \frac{\mu}{\rho} = \frac{1.983 \times 10^{-5}}{1.205} = 1.65 \times 10^{-5}\ \text{m}^2/\text{s}$

That is approximately 16.5 cSt because 1 m2/s = 1,000,000 cSt.

Unit Conversions

Dynamic Viscosity

Reference: Water at 20.2 °C (68.4 °F) has a dynamic viscosity of 1 cP.

Kinematic Viscosity

Reference: Water at 20 °C has a kinematic viscosity of approximately 1.0 cSt.

Viscosity of Common Liquids

Kinematic Viscosity vs. Saybolt Seconds

Kinematic Viscosity vs. Temperature

The original source includes a chart image for kinematic viscosity as a function of temperature. The interactive chart below preserves representative curve readings for common fluids and gases so the trends can be inspected numerically; use manufacturer data for design-critical interpolation.

Fluid Behavior Types

Not all fluids follow Newton's linear stress–strain-rate law. Several categories describe non-Newtonian behavior:

Restored Original Source Tables

The following tables are restored from the original source page to preserve the complete reference data.

Liquids - Absolute Viscosities

Common Liquids - Viscosities

Original Source Images

The following original source images are preserved to avoid losing visual reference material. When an image contains chart or tabular data, its extracted values are represented in the page tables, calculators, or interactive charts; remaining images are retained as visual source references.

Engineering Notes

• Temperature is critical. For liquids, viscosity decreases as temperature rises. For gases, viscosity increases with temperature. Always report or look up viscosity at a specified temperature.
• Reference temperatures in ISO 8217: Residual fuels are characterized at 100 °C; distillate fuels at 40 °C.
• Selecting the right viscosity type. Use dynamic viscosity when calculating shear forces, pressure drops, or drag. Use kinematic viscosity for open-channel flow, lubrication specifications, and fuel grading.
• Unit consistency. Mixing SI and imperial viscosity units is a common source of error. When using the kinematic viscosity formula $\nu = \mu / \rho$, ensure both $\mu$ and $\rho$ use consistent unit systems.
• Glass viscosity. The extremely high viscosity listed for glass reflects near-solid amorphous behavior at room temperature; molten glass at processing temperatures (1000–1400 °C) has viscosities many orders of magnitude lower.
• Measurement methods. Dynamic viscosity is commonly measured with rotational viscometers (Brookfield, cone-and-plate). Kinematic viscosity is measured with capillary (Ubbelohde, Cannon-Fenske) or efflux (Saybolt, Redwood) viscometers.

References

• Engineering Toolbox — Dynamic, Absolute and Kinematic Viscosity