The thermodynamic description of vacancies in binary alloys was expanded to high-entropy alloys (HEAs) within the framework of the CALPHAD method and was applied to estimate the vacancy fraction and the vacancy formation enthalpy in the HEA CoCrFeMnNi. Using the average value of the vacancy formation enthalpy in the pure elements and the excess Gibbs energies in the binary subsystems, the formation enthalpy in the HEA was found to be 1.7 eV. This value is in good agreement with the experimental value of 1.72 eV determined by the positron annihilation technique. This result suggests that the sluggish diffusion cannot be explained from the viewpoint of vacancy formation enthalpy.