A simple formula is proposed for the definition and the calculation of the highest acceptable selectivity coefficient values of ion-selective electrodes. On the basis of the formula which is based on the consideration of the precision of the direct potentiometric measuring techniques one can easily decide whether the selectivity of an electrode is sufficient or not for solving a well defined analytical task correctly. On the other, hand it indicates clearly for ion-selective electrode developing teams or manufacturers how far the ion-selectivity of an electrode should be improved for a certain type of analytical application.