The efficient functioning of bathroom drainage systems is critical for maintaining hygienic and odor-free living spaces. Central to these systems is the gully trap, which plays a key role in preventing the escape of sewer gases while allowing wastewater to flow freely. This study employs Computational Fluid Dynamics (CFD) to analyze the internal flow characteristics within a gully trap, focusing on the impact of varying inlet and outlet diameters on velocity and pressure distribution. Using the SimFlow software, simulations were conducted to visualize and quantify the flow behavior within the trap. The study involved generating an unstructured mesh with a maximum node limit of 200,000 to ensure a detailed capture of flow dynamics. The finite volume method (FVM) and appropriate turbulence models, such as k-ε or k-ω, were utilized for accurate simulations. Verification through grid independence tests and sensitivity analyses ensured the reliability of the results. The findings revealed significant velocity and pressure distribution variations based on changes in inlet and outlet diameters, providing insights into optimizing gully trap design for enhanced performance. This study contributes to environmental engineering by offering a deeper understanding of fluid dynamics in drainage systems, paving the way for developing more efficient and reliable gully traps. The insights gained are expected to aid in reducing blockages and improving water flow efficiency, ultimately enhancing the sustainability and effectiveness of plumbing systems.