We propose a model for predicting the depletion zone arising next to grain boundaries during non-equilibrium segregation. The model directly links to distribution of segregation energies as provided by atomistic simulations. We expose the theoretical framework based on the thermodynamic extremal principle and propose an efficient algorithm to solve the underlying equations. The example of the W-25at%Re is discussed to illustrate the main features of the model. Multi-component segregation kinetics is discussed for segregation of B, C, and N in Mo to illustrate site-competition scenarios. Comparison with earlier results obtained without depletion illustrates the importance of this effects. Finally the depletion zones along GBs are investigated for many different compositions to asses for which material composition and heat treatments they may be observed experimentally. We find that extended depletion zones arise for very small solute concentrations.