The research regarding side weirs of stormwater storage systems which can be considered as effective hydraulic structure to mitigate the urban flooding and estimation of overflowing discharge over the side weir into those storage systems are significant. The De Marchi’s equation seems appropriate for the open channel flow condition to understand hydraulic behavior of the side weir but there are no studies that identify its suitability under the pressurized flow condition. Hence in this study, the overflow discharge coefficient in the De Marchi’s equation is evaluated for pressurized flow condition with different side weir length and height to verify the variation of discharge coefficient. The process of proposing discharge coefficient for each different side weir condition in circular channel is discussed through comparisons between experiment and simulation.