The most common correlations for calculating the friction factor in rough and smooth pipes are reviewed in this paper. From these correlations, a series of more general equations has been developed making possible a very accurate estimation of the friction factor without carrying out iterative calculus. The calculation of the parameters of the new equations has been done through non-linear multivariable regression. The better predictions are achieved with those equations obtained from two or three internal iterations of the Colebrook-White equation. Of these, the best results are obtained with the following equation: 1/rootf = -2.0 log(epsilon/D/3.7065 - 5.0272/Re log(epsilon/D/3.827 - 4.567/Re log((epsilon/D/7.7918)(0.9924) + (5.3326/208.815 + Re)(0.9345)))).