The quantity of Potential for Potable Water Savings (PPWS) made using rainwater harvesting differs between municipalities within the same region. Three main parameters contribute to this difference: roof area, the number of inhabitants and rainwater use scenarios. This article proposes a spatial and numerical analysis of how these parameters influence PPWS across municipalities in the Paris agglomeration. A comparison between the location of the municipalities, the PPWS value and the values for the previous parameters was used to understand the observed difference. The spatial analysis shows that municipalities (districts) in the Paris have the highest PPWS values, on the contrary, the peripheral municipalities in the outer suburbs have the lowest PPWS values and the results for the intermediary municipalitiesin the inner suburbsare contrasting. The numerical analysis demonstrates a strong correlation between the main parameters influencing the PPWS value; where a high value for the roof area means a high value for the number of inhabitants and a good mix of rainwater use scenarios, and vice-versa. In conclusion, the planning and design of the municipalitybuilding density, types of buildings, number of inhabitants, the city's vocationare key factors in defining the level of PPWS made using rainwater harvesting.