Description: The paper presents a mathematical model for calculating the rules of energy and protein in growing pigs and fattening. Were used functions and parameters in the literature (Whittemore, Kyriazakis and Emmans), including our results. Based on this model is shown a procedure for calculating the feed rations. The method of mathematical modelling of the metabolic processes offers the possibility to assess feed allowances in their evolution, related both to the growth rate and to carcass quality