In this paper the squared mass of the hadron is defined as a random variable, whose average is the measured quantity. This leads to a mass formula, of a unique type for mesons and baryons, with a general law for the spin variation of the coefficients. The central squared masses form an overall geometrical scheme; in the baryon case it contains trajectories which are a fine structure of the Regge trajectories. For the accurately measured masses the difference between the computed and experimental value lies within 5 M e V for mesons and baryons. The geometrical scheme will be the basis for the computation of the decay rates, to be developed in the next paper.