Urban water infrastructure is increasingly expected to be resilient to change. To support such resilience goals of cities we propose an approach, which quantifies resilience based on observed or simulated system performance and a tolerable threshold of performance. The approach is demonstrated for the performance of urban drainage systems during storm events regarding their impact on receiving surface waters. The exemplary application underlines that resilience can be quantified and that it may support the understanding of system performance. Moreover, different disturbances (such as storm events or technical system failures) can be assessed separately or in combination. The presented approach is suggested as a starting point to be tested and developed further. In order to allow this development, all the functions used were joined in an R package and made freely available online.