Buckling under action of external lateral pressure of the cylindrical shell stiffened by identical rings with rectangular cross-section sections and a non-stiffened shell of a neutral surface having the same sizes and made of the same material is considered. It is supposed, that the stiffened and non-stiffened shell lose stability at identical critical pressure. Using the simple asymptotic formulas for critical pressure the approximate relations for calculation of optimal stiffened shell parameters corresponding to the minimal value of its weight are received in closed form. The examples of calculations of optimal parameters are presented. Results of the paper may be used at designing thin-walled structures.