Increasing awareness of the health, environmental, and geopolitical issues associated to mineral fertilizers have brought renewed attention to alternative sources of nutrients, such as those that flow from the sanitation systems of cities. This raises the questions: how much can we recover from source-separated human urine and feces? Where are the largest deposits? Where should we start? To answer these as precisely as possible—taking into account the specificity of the area of interest, such as local diets or population pyramid—this work proposes a detailed model of the local territorial metabolism and the associated flows of nitrogen and phosphorus. As it can be adapted for any country to assess deposits at various scales, from national down to the individual building, it can support both policymakers and local urban planners. The model consists in the combination of equations describing human metabolism, information on the local diet, and open statistics on the populations inhabiting the area. Its results are validated against sanitation data from two areas in France and the Netherlands and compared to previous works in the scientific literature. We also demonstrate how the model can be used to inform prospective analyses on the evolution of the load entering sanitation systems and the nitrogen-to-phosphorus ratios, depending on population and dietary changes. Finally, we discuss how this open-source model can assist local actors in planning projects to separate human excreta and produce local and recycled fertilizers to increase their food security and autonomy.