Integrated interdiffusion coefficient, introduced by Carl Wagner in 1969, is revisited. First, it is applied for the whole diffusion zone, consisting of solid solutions and/or layers of intermediate compounds. We demonstrate, for the first time, that the Wagner coefficient satisfies the simple additive rule: the total squared inter-penetration width is proportional just to the sum of all Wagner coefficients of all intermediate phases of the system. Second, we check the applicability of Wagner coefficients in the atomistic-scale modelling of reactive diffusion. Checking is done for the growth of intermediate compound AB with BCC lattice.