text: The phenomenological Colebrook–White equation (or Colebrook equation) expresses the Darcy friction factor f as a function of Reynolds number Re and pipe relative roughness ε / D, fitting the data of experimental studies of turbulent flow in smooth and rough pipes.
  The equation can be used to (iteratively) solve for the Darcy–Weisbach friction factor f.
  For a conduit flowing completely full of fluid at Reynolds numbers greater than 4000, it is expressed as:
  or
  where:
  * Hydraulic diameter,  (m, ft) – For fluid-filled, circular conduits,  = D = inside diameter
  * Hydraulic radius,  (m, ft) – For fluid-filled, circular conduits,  = D/4 = (inside diameter)/4
  Note: Some sources use a constant of 3.71 in the denominator for the roughness term in the first equation above.