The paper deals with the tray-to-tray method for determining the required number of trays in gas-liquid columns in case of intensive entrainment. The equations for estimation of the tray efficiency are derived for top, bottom and intermediate trays, and they incorporate the number of gas and liquid phases transfer units, stripping factor, Peclet number of mixing and fractional entrainment. These equations can be simplified to the well-known one-parametric diffusion model explained in Bubble Tray Design Manual (1958) [Bubble Tray Design Manual, 1958, AIChE, New York] for case of negligible entrainment. The algorithm of tray-to-tray method is based on tray efficiency calculated for each tray separately and it enables the calculation of the number of required trays, starting from the bottom tray. Given example shows how the excessive entrainment decreases of the driving force of the process. (c) 2007 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.