Six sampling points located at different distances from the main water reservoirs, were selected in Benha Water Supply Network (BWSN), Egypt, in order to follow the evolution of total trihalomethane (THMs). The gained information from the monthly sampling and laboratory analysis program was used to formulate relationship of total trihalomethanes (THMs) concentration in terms of initial chlorine dose, total organic carbon, bromide ion concentration, contact time, temperature, algae, and pH. To define the accuracy level of the developed model, the predicted THMs concentrations from the developed model were compared to actual THMs concentrations measured during the monitoring period. The developed mathematical model and software (EPANET), which was used to determine water age, were used to predict THMs concentrations throughout BWSN. The results showed close agreement between the measured and calculated THMs concenteration. Hence the difference in the average calculated values and measured values during the study period ranged from (4.6% to 9.9 %). The model can provide reasonable results for most of the functioned variables and can be used to have a rapid assessment of the trihalomethanes formation