Abstract <p>Gas–liquid systems under the effect of gravity are separated in gravity separators, which, with sufficiently high efficiency, are structurally simpler and also create a lower pressure drop compared to cyclone separators. There are various domestic and foreign methods for calculating horizontal hollow gas–liquid separators. They have such disadvantages as applicability only to certain deposition modes, excessive simplification of the applied models, difficulty in automating the calculations, etc. The identification of these shortcomings makes it possible, based on the theory of deposition and the description of the geometry of the circular section of a horizontal gas–liquid separator, to develop a methodology in which the process of droplet deposition is described by a universal criterion equation and the degree of filling of the apparatus with liquid, affecting the height of droplet deposition, is also taken into account. The obtained method makes it possible to determine the product of the length of the separation zone by the separator radius and, based on this, select a typical device. The development of a new universal technique increases the reliability of the design of devices and their subsequent operation.</p>