In fluid dynamics, it is sometimes more appropriate to work in terms of kinematic viscosity (sometimes also called the momentum diffusivity), defined as the ratio of the dynamic viscosity () over the density of the fluid (). It is usually denoted by the Greek letter nu ():
and has the dimensions , therefore resulting in the SI units and the derived units:
: <math>  
  [\nu]
= \mathrm {\frac {m^2} {s}} 
= \mathrm {\frac {N{\cdot}m} {kg} {\cdot}s}
= \mathrm {\frac {J}{kg} {\cdot}s}
 energy per unit mass multiplied by time.