Three future projections of global municipal water use are established: business-as usual (BAU), low technological improvement (Low Tech), and high technological improvement (High Tech). A global municipal water demand model is constructed using global water-use statistics at the country scale, calibrated to the base year of 2005, and simulated to the end of the 21st century. Since the constructed water demand model hinges on socio-economic variables (population, income), water price, and end-use technology and efficiency improvement rates, projections of those input variables are adopted to characterize the uncertainty in future water demand estimates. The water demand model is linked to the Global Change Assessment Model (GCAM), a global change integrated assessment model. Under the reference (BAU) scenario, the global total water withdrawal increases from 466 km3 year−1 in 2005 to 1098 km3 year−1 in 2100, while withdrawals in the High and Low Tech scenarios are 437 and 2000 km3 year−1, respectively.