English: A graphical depiction of the relationship between S = Δ p / L, the pressure loss per unit length of pipe, versus flow volume Q, for a range of choices for pipe diameter D, for air at standard temperature and pressure. Units are SI. Lines of constant flow velocity V and lines of constant Reynolds number Re are also shown. The relationship is the Darcy-Weisbach equation with the Darcy friction factor given by the Colebrook-White equation.