Nuclear spallation reactions are treated as two step processes involving energy deposition and subsequent evaporation. The first step is calculated in Glauber's multiple scattering formalism with modification for direct knockout processes. Evaporation is treated by following the mean yield of particles from the residual nucleus taken as an excited Fermi gas with ground state provided by the semiempirical mass formula. The spallation mass yield is derived to be an exponential in the mass of the fragments. The amplitude of the exponential and its slope depend on the projectile energy and target mass. Experimental data are well reproduced without free parameters.