The Langelier and the Stiff-Davis saturation indices have found wide acceptance as indicators of the degree of water saturation with respect to calcium carbonate. But the risk of aggressiveness. or scaling may be accurately evaluated from the analytical data by application either of the graphic methods or the complex methods of calculation made possible by the use of data processing. The present work was devoted to the study of this problem using a computerized model of Legrand and Poirier, based on the plot of the CO2 concentration vs. Ca2+ concentration. Taking into account the formation, in natural waters, of ion pairs as well as the various equilibrium constant variation with temperature made improvements to this model. Consequently the ionic activity coefficients were calculated using suitable models. Calculation was developed by means of the graphical LabView(TM) programming language. The calco-carbonic curves obtained constitute a useful tool to visualize if the studied water was in equilibrium or not and provide quantitative indications of how much reagent, such as lime or soda, must be added to bring water to equilibrium. Calculations carried out with the program were illustrated by some results and compared with those of Legrand and Poirier and others studies.