A model of the lower operating limits of distillation and absorption trays is described in this paper. The model requires the simultaneous solution of two equations describing ( 1) liquid flow across the tray and over the outlet weir, and (2) countercurrent liquid flow through the free hole area of the tray. The second equation stems from models of a tray without downcomers; this equation was validated by means of available test data. The model gives liquid height and weep fraction as a function of gas and liquid flow rates, for a given tray layout. As shown, it permits calculation of the gas flow rate at weep and seal point, in a coherent way. Examples illustrate the use of the method in calculating the position of the weeping range of a tray and its relation to tray efficiency.