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Dynamic Vs Kinematic Viscosity

Choose the correct viscosity property for Reynolds-number, pressure-loss, and fluid-data workflows without mixing density into the wrong place.

Referencefluids & pipingLast reviewed: 2026-07-29

Use this distinction when a fluid datasheet gives viscosity in cP or cSt, or when two versions of a Reynolds-number equation appear to require different inputs.

What This Means

Dynamic viscosity describes a fluid's resistance to shear. It is the property represented by mu in the shear relationship and in the density-based Reynolds-number form.

Kinematic viscosity is dynamic viscosity divided by density. It packages the shear resistance and density effect into one property, which is why the alternate Reynolds-number form does not contain density explicitly.

Neither property is universally interchangeable with the other. The conversion is valid only when viscosity and density refer to the same fluid state, especially the same temperature and pressure.

Key Relationships

nu = mu / rho
mu = rho nu

Re = rho V D / mu
Re = V D / nu
  • mu is dynamic viscosity, commonly expressed in Pa*s, mPa*s, or cP.
  • nu is kinematic viscosity, commonly expressed in m^2/s or cSt.
  • rho is fluid density.
  • V is average velocity and D is the characteristic diameter.
  • 1 cP = 1 mPa*s and 1 cSt = 1 mm^2/s.

The two Reynolds-number forms are algebraically identical when nu = mu / rho is applied consistently.

Use This When

  • Matching a supplier property sheet to the viscosity input on a pipe-flow calculation.
  • Converting an oil viscosity reported in cSt into dynamic viscosity for a pressure-loss method that also uses density.
  • Checking whether a Reynolds-number worksheet expects mu or nu.
  • Comparing fluid properties across operating temperatures.
  • Reviewing why two calculations disagree even though they appear to use the same viscosity value.

Assumptions

  • Dynamic viscosity, kinematic viscosity, and density describe the same fluid composition and state.
  • The fluid can be treated as Newtonian for the calculation being performed.
  • Reported viscosity units are known; a bare viscosity number without units is not sufficient.
  • Any temperature interpolation is appropriate for the source data and operating range.

Limitations

  • Viscosity can change strongly with temperature, and gases and liquids do not follow the same temperature trend.
  • Non-Newtonian fluids require a shear-rate-dependent model rather than one constant viscosity value.
  • Mixtures, slurries, emulsions, and polymer solutions may not be represented adequately by a generic fluid-property value.
  • Converting between mu and nu does not correct an unsuitable viscosity measurement or extrapolation.
  • High-pressure service may require pressure-dependent property data.

Common Mistakes

  • Entering a cSt value into a field expecting cP because both are called viscosity.
  • Multiplying or dividing by density twice when switching between Reynolds-number forms.
  • Using density and viscosity from different temperatures.
  • Treating Pa*s and mPa*s as the same magnitude.
  • Assuming a room-temperature water or oil value applies at the actual operating temperature.

Sources

This reference uses White's Fluid Mechanics for the definitions of dynamic and kinematic viscosity and their Reynolds-number forms. Crane TP-410 corroborates the pipe-flow use of viscosity, density, and Reynolds number in practical loss calculations.

  1. Frank M. White. Fluid Mechanics, 7th ed., McGraw-Hill, 2011. ISBN 978-0-07-352934-9.
  2. Crane Co.. Flow of Fluids Through Valves, Fittings, and Pipe (Crane TP-410), Technical Paper No. 410, Crane Co., 2009. TP-410.