text: Moody's team used the available data (including that of Nikuradse) to show that fluid flow in rough pipes could be described by four dimensionless quantities: Reynolds number, pressure loss coefficient, diameter ratio of the pipe and the relative roughness of the pipe.  They then produced a single plot which showed that all of these collapsed onto a series of lines, now known as the Moody chart. This dimensionless chart is used to work out pressure drop,  (Pa) (or head loss, (m)) and flow rate through pipes. Head loss can be calculated using the Darcy–Weisbach equation in which the Darcy friction factor  appears :
  Pressure drop can then be evaluated as:
  or directly from
  where  is the density of the fluid,  is the average velocity in the pipe,  is the friction factor from the Moody chart,  is the length of the pipe and  is the pipe diameter.
  The chart plots Darcy–Weisbach friction factor  against Reynolds number Re for a variety of relative roughnesses, the ratio of the mean height of roughness of the pipe to the pipe diameter or .
  The Moody chart can be divided into two regimes of flow: laminar and turbulent. For the laminar flow regime (was determined analytically by Poiseuille:
  For the turbulent flow regime, the relationship between the friction factor  the Reynolds number Re,  and the relative roughness is more complex. One model for this relationship is the Colebrook equation (which is an implicit equation in ):