Viscosity Converter: Dynamic & Kinematic

Kinematic vs Dynamic Viscosity: What is the difference?

If you are working in fluid dynamics, lubrication engineering, or chemical manufacturing, you will frequently encounter the debate of kinematic vs dynamic viscosity. While both measure a fluid's resistance to flow, they look at it from two different physical perspectives.

The core difference lies in density. Dynamic viscosity measures the absolute force required to move a fluid, while kinematic viscosity measures how fast that fluid flows under the weight of its own gravity.

What is Dynamic Viscosity?

So, what is dynamic viscosity? Also known as absolute viscosity, it is the measure of a fluid's internal resistance to deformation or flow. Imagine stirring a cup of honey versus a cup of water—the honey requires much more physical force to stir because it has a higher dynamic viscosity.

What are the units of dynamic viscosity? The most common units of dynamic viscosity are the Poise (P) or Centipoise (cP). In the SI (metric) system, it is measured in Pascal-seconds (Pa·s) or millipascal-seconds (mPa·s). Conveniently, 1 cP is exactly equal to 1 mPa·s.

What is Kinematic Viscosity?

Conversely, what is kinematic viscosity? It is the ratio of dynamic viscosity to the density of the fluid. Instead of measuring the force required to move the fluid, kinematic viscosity measures the time it takes for a fixed volume of fluid to flow through a capillary tube under the force of gravity alone.

The standard units for kinematic viscosity are the Stokes (St) or Centistokes (cSt). In the SI system, it is measured in square millimeters per second (mm²/s). Again, 1 cSt is exactly equal to 1 mm²/s.

How to Calculate Dynamic and Kinematic Viscosity

If you need to know how to calculate dynamic viscosity or how to find kinematic viscosity, you must know the fluid's density. The mathematical relationships are straightforward:

Using these formulas, you can easily convert between cSt and cP as long as your density is expressed in g/cm³ or Kg/L.

Real World Examples: Water and Air

To understand these concepts, let's look at the benchmarks: water and air.

Dynamic Viscosity of Water

What is the dynamic viscosity of water? At room temperature (20°C / 68°F), the dynamic viscosity of water is famously close to 1.002 cP. Because the density of water is essentially 1 g/cm³ at this temperature, the kinematic viscosity of water is also approximately 1.004 cSt. This 1-to-1 ratio makes water the universal baseline for all fluid dynamics.

Dynamic Viscosity of Air

Gases are fluids, too! The dynamic viscosity of air at 20°C is extremely low—about 0.0181 cP. However, because air is incredibly light (a density of just 0.0012 g/cm³), its kinematic viscosity is actually very high (around 15.1 cSt). This means that while air is easy to push (low dynamic), it flows very sluggishly under its own practically non-existent weight (high kinematic).