Many water authorities have promoted the installation of water-saving devices as an effective way of managing residential water demand. However, there has been some controversy about the effectiveness of such devices in studies in developed countries, and few studies have considered the use of water-saving devices in developing countries, where a rapid increase in water demand is expected. This study focused on toilet replacement and the residential water-saving potential in urban Hanoi, Vietnam – a country presently enjoying remarkable economic growth and increasing water demand – using the Monte Carlo simulation, i.e., stochastic, approach. We surveyed the toilets of 134 households. Annual water savings were not large – 2.2 liters per person per day (1.9% of total residential consumption) – despite the assumption that all toilets would be replaced with efficient ones within three years. Based on this study, a rebate to encourage toilet replacement would have limited effect for saving water, contrary to experience in developed countries. Comparing the volume of water saved, the simple analytical model, i.e. the deterministic approach, under-estimated the water-saving potential.