The overview of the experimental and theoretical data on the ionization energies (IE) of the Cu -like ions is presented. In various works data are obtained by interpolation and extrapolation the parameters of model potentials for Dirac equations. In other approaches, the function of the dependence of the ionization energy on Z (Z is nuclear charge) is approximated by a polynomial with power-law dependence on Z in order to achieve a minimum difference between theoretical and experimental data. The compilation of ionization energies is in database of the National Institute of Standards and Technology (USA), where the typical uncertainty is several units in the fourth significant digit. This means that for heavy ions the error can reach several tens of thousands cm–1. In this work, the ionization energies of Cu-like ions are refined to achieve accuracy to the fifth significant digit. Two methods have been developed for the interpolation and extrapolation. Scaling ionization energies along Z: scaling results in the function of the dependence of ionization energies on Z to the form of a quasi-line, i.e., weakly varying function on interval of 10−15 Z values. This allows the function to be interpolated up to the fifth significant numbers. The scaled refined function of the dependence of the ionization energy on Z is approximated by analytical functions that allow extrapolation with good accuracy to the Z~92 region. The relativistic model potential is used to interpolate and extrapolate ionization energies. The parameter of the relativistic model potential for the 4s1/2 orbital turned out to be almost linear function of Z for Z > 70, which made it possible to extrapolate with high accuracy to the Z region of 92. The results of both methods are in good agreement up to Z~ 92.