The process of estimating urban water demand is considered as a rational, technical and apolitical process based on a set of standardised principles and therefore it has received the least attention from critical sociologists, urban geographers and STS scholars. The paper examines the history of piped water supply in Mumbai municipal corporation and illustrates how the process of demand estimation is used as a tool to legitimise the (mis)appropriation of water resources leading to inequitable distribution of water among the cities and villages belonging to Mumbai Metropolitan Region (MMR). Historically, engineers and city planners are using higher per capita water consumption standards and unreasonable assumptions and manipulating the demand estimation process to produce unrealistic numbers of higher water demand. Using such numbers, the engineers have constructed the narratives of water shortages and successfully mobilised the resources for the construction of new dams and diversion of water reserved for irrigation from the existing dam. It reflects the anxiety among engineers towards bringing more and more water for the city and also securing the water sources for the future. By doing such manipulations, the engineers are also hiding the inefficiencies of the water supply system which they have designed and managed over the years. In addition, the paper raises questions over the objectivity and validity of per capita consumption standard - a founding stone for water demand estimation process. In the case of Indian cities, the per capita standard is prescribed by multiple agencies and revised in an ad-hoc fashion without citing any theoretical or empirical rationale. The irony is though with high per capita standards Mumbai is capturing water from the region, there is no technical provision in the distribution network to ensure that each individual is receiving her share. This reveals how engineers control water resources in urban agglomeration.