When water shortages occur, toilet water-saving becomes an effective way to solve the problem. We developed a new method that utilizes the accumulated flow rate to predict the flushing performance and to obtain design data for the development of high-efficiency toilets. The results showed that the length and shape of the trapway are the most important factors for flushing performance, and additional water saving can be achieved by delaying the opening of the trapway outlet valve. In addition, the tank water level in the water tank needs to be optimized considering siphon intensity as well as siphon duration. In conclusion, the toilet was optimized up to 6.5 kg of water using 60 mm diameter flexible tube by the method of accumulated flow rate measurement. In the future, the development of an ultra high efficiency toilet below 4.5 kg will be tried utilizing the method.