The monograph summarizes the physical principles of using the concept of "demagnetizing factor" in the calculations of the process and the result of magnetization of a ferromagnetic body. A logical chain has been built from classical calculations of the demagnetizing factor of ellipsoids to interpolation formulas for calculating the demagnetization coefficients of inhomogeneously magnetized bodies. Formulas for calculating the demagnetizing factor of products of different shapes under different magnetization conditions have been substantiated and developed. Efficiency of their use in solving problems of magnetic structural analysis and magnetic measurements is shown.