The formation of a butt weld bead occurs at the conjoined pipes during the process of butt-welding for pipelines in water and wastewater networks. Pressure losses in pipes with weld beads are often underestimated in design practice that affects the safety and reliability of the system. This paper focuses on the effect of weld beads and pressure loss in polyethylene (PE) pipes using computational fluid dynamics (CFD) simulations. The CFD model is first validated against the theoretical results and previous weld bead pressure loss experimental data. Both k-ε and k-ω turbulent models are examined, confirming k-ω shear stress transport (SST) model agree better with the theoretical pressure loss, which was considered for all inlet Reynolds numbers. The CFD simulations are conducted for long PE weal bead pipes with varying pipe diameters. The results confirm that as the pipe diameter increases the local head loss increases although the difference between 500 mm and 800 mm at lower inlet Reynolds number is marginal. Finally, the effect of weal bead shape was studied for the 800 mm pipe. Among bead’s height, width and cleavage depth, the bead’s height has a more significant impact, demonstrating that the pressure head loss is minimised when the bead’s height is minimum. The results confirm the head loss can be reduced by 5% for optimised weld bead shape, ensuring the water network is more cost-effective and reliable.