Growth of urban population around the world and, particularly, within urban areas, has placed various pressing challenges on Food, Energy, and Water (FEW) such as food security, water safety as well as energy scarcity and so on. Current studies on urban FEW nexus are mainly focused on the correlation analysis of elements by pairs, while these works are developed separately. With respect to the methods, the existing researches mostly adopt the bottom-up approach, accounting for the direct relationship between the individual production sectors. While the associations between the internal elements of the system still lack of simulation. In this study, we aim at developing an online open access tool for cities, the Urban Circular Economy Calculator (UCEC), which enables to develop different circular economy scenarios associated to FEW management. UCEC v1.0 uses Beijing data as test case. In particular, more than 20 circular economy policies related on food, energy and water are selected and divided into 6 categories. Long-term simulations on the social, economic and environmental impacts are provided to test the trajectories of policy effects. Being an open access tool, UCEC can be used also for supporting participatory processes as an urban management instrument. The solution is economically and financially feasible, due to the low level of technical requirements. The necessity of such a tool is proved by the societal need of transition toward a low-carbon and sustainable framework, which can be effectively supported by the introduction of circular economy. This transition, such as the idea behind UCEC, should preserve (or even improve) the societal wellbeing, while increasing basic resources (i.e.: FEW) accessibility, security and preservation.