Weirs are structures that are important for measuring flow and controlling water levels. Research has shown that the discharge coefficient is not constant and depends on the crest length, the height of the weir, the upstream head, and the upstream and downstream slopes. In this study, the effect of these parameters on the discharge coefficient (C_d) is investigated by numerical simulation. The current study present numerical simulation using the ANSYS FLUENT software. The total number of simulations is 432 which includes: 4 upstream slopes, 4 downstream slopes, 3 weir heights, 3 upstream heads (h₁) and 3 weir crest lengths. It was found that the downstream face slope has little effect on C_d. For 0.1 < H₁/w < 0.4 by decreasing the upstream slope, C_d increases, where H₁ is the water head on the weir crest and w is the length of the crest. Also, for the same range, by decreasing the height of the weir (p), the C_d increases. For 0.16 < H₁/p < 2, as the length of the crest decreases, the C_d increases. By comparing the numerical simulation results to physical measurements, multi-variable regression equations for estimating C_d are presented. In addition to C_d, extraction of other more detailed information such as water level profiles and velocity profiles at different locations is provided.