With increasing global attention on environmental issues, the construction industry is seeking more sustainable building materials and methods. Engineered Cementitious Composite (ECC), used as permanent formwork, can significantly reduce carbon emissions from discarded temporary formworks while enhancing the seismic performance and safety of building structures. However, the high cost of ECC materials poses challenges for their application in engineering. This study aims to investigate the optimal formwork thickness for ECC permanent formworks under the ideal failure mode, where the steel reinforcement yields first, followed by the crushing of the concrete beam. The study calculates the ultimate loads for both the steel reinforcement and the concrete beam, selecting the formwork thickness where the ultimate load of steel reinforcement yielding is slightly less than that of concrete beam crushing as the optimal formwork thickness. The Ultimate load-bearing capacities of RC composite beams with varying formwork thicknesses of 15, 20, 25, and 30 mm are calculated for beams with the same reinforcement ratio and approximate span-to-depth ratio. Optimal web thicknesses for span-to-depth ratios of 5, 6.67, 8, 10, and 13.33 are determined. Additionally, finite element analysis using ABAQUS is conducted to validate the optimal web thickness for a span-to-depth ratio of 13, confirming the agreement with the calculated results. It is recommended that a formwork thickness of 30 mm be used for composite beams with span-to-depth ratios less than 8 and 15 mm for those with ratios greater than 12. However, these findings are specific to beams, and further investigation is required to assess their applicability to columns, slabs, and other structural elements.