Also called Colebrook-White. It is the classic formulation for [[Darcy friction factor]]. However it is implicit and needs iteration $\frac{1}{\sqrt{f_D}}=-2log_{10}\left[\left(\frac{\varepsilon /D}{3.7}\right) + \frac{2.51}{Re\sqrt{f_D}}\right]$ It was developed by Colebrook in 1939 extending work of Nikuradse who was a student of Prandtl It was been plotted by Moody in 1944 into what is known as [[Moody chart]] Since it needs to be iteratively solved, Haaland provided a simplication called [[Haaland equation]] which can be explicitly calculated