An empirical equation for the number of entanglements per molecule has been proposed, which applies over all the molecular weight range. On this ground a simple equation for the zero shear viscosity of monodisperse polymer melts, eta(0), has been worked out that appears able to properly take into account the sharp transition of viscosity between the monomeric and the entanglement regimes. The molecular parameters appearing in the new viscosity equation are: the monomeric molecular weight m(0), the monomeric friction factor zeta(0), the molecular weight M, the average molecular weight between entanglements M-e, and the entanglement friction factor zeta(e)(3.4). This last parameter was evaluated for a number of monodisperse polymers.