Kinematic Viscosity

Kinematic viscosity (ν) is dynamic viscosity (μ) divided by fluid density (ρ): ν = μ/ρ. It describes viscosity relative to density, so converting from dynamic to kinematic viscosity requires the density of the fluid at the same conditions.

Kinematic-viscosity units

The SI unit is square metres per second (m²/s). Other units include square millimetres per second (mm²/s), stokes (St), and centistokes (cSt). One cSt equals 1 mm²/s, and 1 St equals 100 cSt.

How it differs from dynamic viscosity

Dynamic viscosity uses units such as Pa·s, mPa·s, poise (P), and centipoise (cP). These measure a different quantity from kinematic-viscosity units such as cSt; cP and cSt are not interchangeable by a fixed factor. Density is required to relate them.

Example

If dynamic viscosity is 100 cP (0.1 Pa·s) and density is 850 kg/m³, then ν = 0.1/850 = 0.000117647 m²/s, or about 117.647 cSt (117.647 mm²/s).