This paper describes a methodology and a fully tested and calibrated mathematical model for the treatment of endwall effects in axial compressor aerodynamic calculations. Additional losses and deviations caused by the clearance and secondary flows are analyzed. These effects are coupled with endwall boundary layer losses (EWBL) and blockage development. Stall/surge detection is included, and mutual interaction of different loss mechanisms is considered. Individual mathematical correlations for different effects have been created or adopted from earlier papers with the aim of forming one integral model that is completely described in this paper. Separate mathematical correlations and calibration measures are discussed in detail in the first part of the paper. The developed overall model is suitable for application in two-dimensional (2D) or mean-line compressor flow calculations. During the development, it was tested, calibrated, and validated using throughflow calculations comparing numerical results with experimental data for a large number of test cases. These test cases include compressors with very different configurations and operating ranges. The data on the compressors were taken from the open literature or obtained from industrial partners.