In practice, drinking water technologists use simplified calculation methods for aquatic chemistry calculations. Recently, the database stimela.dat is developed especially for aquatic chemistry for drinking water treatment processes. The database is used in PHREEQC, the standard in geohydrology for calculating chemical equilibria in groundwater. The development of a graphical user interface for PHREEQC in Microsoft Excel has made it possible to easily incorporate complicated chemical calculations for use by technologists of drinking water treatment plants. By making use of PHREEQC the calculations performed are more accurate because of inclusion of ionic strength, ion pairs and most recent determined chemical equilibrium constants. Due to this development it is possible to for instance validation of laboratory measurements and on-line sensors. The use of PHREEQC is demonstrated in a simulation of the chemical water stability at drinking water treatment plant Weesperkarspel of Waternet.