Abstract: The purpose of this thesis is to display different approaches and methods of determining water quality index. To explain the choice of certain parameters and the determination of parts by weight of each parameter in total figures, using the Velika Gorica water well as an example. Taking into account the advantages and shortcomings of each water evaluating quality method, a water quality index was determined in the Velika Gorica water well area. Using the method of universal water quality index, where chemical analysis results of groundwater from years 2014 and 2015 were used. While calculating the water quality index, the focus was put on choosing parameters and determining their parts by weight. From a total of 94 parameters, 14 were chosen (pH, total hardness, nitrate, iron, magnesium, sodium, potassium, chlorides, sulphates, the total number of coliform bacteria, manganese, electrical conductivity, and atrazine). The most interesting combinations of parts by weight were chosen to showcase the effect of a subjective assessment on the resulting water quality indices. Depending on the combination of parts by weight and the exclusion of certain parameters from the calculation, water quality indices can differ greatly. Therefore, it is important to state that the index itself does not point to the cause of any given situation (for example, bad water quality). Additional information and specific data are needed to come to such conclusion.