Variant calculation is the first step in gas dynamic design. The study provides brief information on the simplified mathematical model of efficiency and basic equations for estimating the efficiency and basic parameters of compressors. It is possible to compare the variants of compressors with a different number of shafts and rotation frequencies, with a different number and location of gas coolers in the flowing part, with different types of impellers and diffusers. Examples of a variant calculations are given for two types of compressors of a linear gas compressor unit and of a compressor for underground gas storage. It is shown that single-shaft compressors for underground gas storages should be optimized by selecting the maximum possible number of stages, by selecting the ratio of loading factors and the diameters of the impellers. It is shown that the single-shaft scheme of a general-purpose air compressor is inferior in efficiency to a two-shaft scheme with a large number of intermediate cooling.