In a cylindrical pipe of uniform diameter , flowing full, the pressure loss due to viscous effects  is proportional to length  and can be characterized by the Darcy–Weisbach equation:
where the pressure loss per unit length  (SI units: Pa/m) is a function of: 
: , the density of the fluid (kg/m);
: , the hydraulic diameter of the pipe (for a pipe of circular section, this equals ; otherwise  for a pipe of cross-sectional area  and perimeter ) (m);
: , the mean flow velocity, experimentally measured as the volumetric flow rate  per unit cross-sectional wetted area (m/s);
: , the Darcy friction factor (also called flow coefficient ).
For laminar flow in a circular pipe of diameter , the friction factor is inversely proportional to the Reynolds number alone () which itself can be expressed in terms of easily measured or published physical quantities (see section below). Making this substitution the Darcy–Weisbach equation is rewritten as
where
:  is the dynamic viscosity of the fluid (Pa·s = N·s/m = kg/(m·s));
:  is the volumetric flow rate, used here to measure flow instead of mean velocity according to  (m/s).
Note that this laminar form of Darcy–Weisbach is equivalent to the Hagen–Poiseuille equation, which is analytically derived from the Navier–Stokes equations.