Spillway design discharge and stage are determined by the routing method chosen, and they are crucial in determining the dam crest level. Because a dam is such a large structure, its height has a substantial impact on its cost estimation, the necessity of optimum spillway capacity and design discharge estimation need worth consideration. The commonly used Modified Pul's routing method, which is widely accepted as a standard method, presupposes that there is a horizontal water surface level above the FRL which controls by the downstream structure. However, upstream boundaries such as the inlet cross sectional geometry, river bed slope, and surface roughness regulate the flow, therefore upstream control prevails at the entrance, and downstream structure controls the water surface level at the exit. As a result, there is a variation in elevation between the upstream and downstream edges, resulting in a sloped reservoir water surface. When we substitute the horizontal water surface of the usual Modified Pul's Technique assumption with this sloped water surface, the reservoir adds an extra storage that the common method does not account for, and this extra storage reduces the out-flow discharge. This research modify the common Modified Pul’s Method which tries to handle and consider this extra storage which ignored in the common method using a MATLAB computer program by taking Kesem Dam as a case study and develop a standalone application. The result shows that there is a reduction in the out-flow discharge and design head through the new modified method.