Water quality is of great importance due to its impact on human health and aquatic systems. Since Islamabadand Rawalpindi get their drinking water from Simly dam, this makes it an important drinking water source. In thisstudy water quality parameters of Simly dam were determined and water quality index (WQI) using Arithmeticweighted method was calculated. Collection of water samples from different locations of dam and the filtration plantinstalled in the facility was done. Twelve physicochemical parameters of water quality including: pH, ElectricalConductivity (EC), Total Dissolved Solids (TDS), alkalinity, hardness, Biological Oxygen Demand (BOD), DissolvedOxygen (DO), Total Chloride, Calcium, Nitrates, Potassium and Sodium (Na) were measured to estimate the sentientand water quality of the Simly dam. WQI of both raw dam water and filtered dam water was determined using WorldHealth Organization (WHO) standards for drinking water. The water quality index of raw dam water was found to be62.79 while that of filtered dam water was 49.43. The WQI of the unfiltered raw dam water was found to be in class Cthat is poor category, while the filtered dam water was falling in Class B that refers to good category according to waterquality index categories. Regular monitoring of water quality is recommended to ensure safe water supply toconsumers.