The overall friction coefficient (K-f) of airlift reactors was estimated using equivalent lengths (L(eq)) and friction factors (f). The friction factor was calculated taking into account the riser liquid velocity profile corresponding to the two-phase flow and using classical one-phase equations. A previously described model was used to obtain simultaneously both gas hold-up and liquid circulation velocity. The model simulates experimental data obtained in a wide range of configurations of internal (2 and 30 dm(3) volume) and external (from 8 to 600 dm(3) volume) airlift reactors with Newtonian (water and alcohol solutions) and non-Newtonian (carboxymethylcellulose (CMC) solutions) systems. Comparison with other models from the literature yielded similar results.