This paper describes a physical model for a stable drop size in liquid pulsed extractors. It is based on the physical phenomena of drop disintegration which were experimentally investigated in a small test setup. The aim was to derive a relatively simple model on the basis of meaningful simplifications and assumptions, however, including the drop phase viscosity and other important influences. A comparison of the calculated and measured drop sizes shows that the new equation given in this paper is able to calculate the diameter of stable drops in the technical range of drop phase viscosities, pulsation intensities and sieve tray parameters.