The purpose of this study is to propose a calculation method for estimating the carrying performance of water-saving toilets installed in commercial buildings, in order to contribute to the design of drainage pipes taking into account the carrying performance. The carrying performance when a large-diameter pipe is installed in water-saving toilets has been determined by actual measurements. However, actual measurements have limitations in adapting to various pipe configurations. Therefore, in this paper, we estimate the carrying performance of straight pipes by calculating the effects of variations in flushing water volume, pipe diameter, and gradient, and evaluate their effectiveness.