The Dean number is typically denoted by De (or Dn). For a flow in a pipe or tube it is defined as:
where
*  is the density of the fluid
*  is the dynamic viscosity
*  is the axial velocity scale
*  is the diameter (for non-circular geometry, an equivalent diameter is used; see Reynolds number)
*  is the radius of curvature of the path of the channel.
*  is the Reynolds number.
The Dean number is therefore the product of the Reynolds number (based on axial flow  through a pipe of diameter ) and the square root of the curvature ratio.