The error of the weak binary electrolyte dissociation degree calculations using the approximate formula \(\alpha = \sqrt {{{{{K}_{c}}} \mathord{\left/ {\vphantom {{{{K}_{c}}} c}} \right. \kern-0em} c}} \) and by the Ostwald dilution law is analyzed. The expressions for the relative error are derived and the limits of application for the specified approximations are set up, in particular, when the solvent self-dissociation must be considered.