Size distribution of crude oil drops in the permeate has been predicted using the 'linear fit' approach with, and without oscillating the membrane. Without membrane oscillation (without shear rate), static and drag forces are taken into account and their balance is assumed as 100% cut-off or rejection point. With the membrane oscillation, 'inertial lift' model available in the literature is considered and in this case 100% cut-off is assumed when away migration and convection velocities becomes equal. Three types of crude oil drops with different API values and interfacial tensions are analysed experimentally, the results are compared with the presented model and the model is an agreement with the experiments. The study has been validated with the genuine size distribution of oil drops obtained from oil companies operating at Kuwait at various locations in order to investigate the industrial applicability of the model. Overall oil concentration of the permeate can be calculated using the proposed model that provides an idea whether the concentration of oil in the permeate is within the standard set by international regulatory authorities. (C) 2014 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.