Trees provide essential ecosystemic services (e.g. stocking carbon or locally regulating temperatures) and play an important role in the resilience to climate change, especially in urban areas. Quantifying these services in cities is difficult because little information is known about each tree, and no allometric model yet exists for urban trees. The UrTrees project calls the citizen to the rescue to help collect measurements and increase our knowledge of urban tree key features. Using the mobile app that we have designed, only a short video around the tree is necessary to approximate three key measurements: the tree height, its diameter at breast height (1m30 above ground) and the crown volume. No expertise in trees is required to use UrTrees, which has even been tested on children from 6 years old. A 3-dimensional point cloud of the scanned tree is first derived from the video using Structure-from-Motion algorithms. Geometric models for the trunk and the crown are then fitted to the point cloud in order to estimate the key measurements. Additionally, Pl@ntNet can be used to identify tree species. All measurements are stored in a database providing data for urban tree studies and feedback to the mobile app user. Efforts have been put into the mobile app user experience, with a scoring system, daily quests and point cloud interactive visualisation. Individual tree information collected through the app will be made freely available to the general public.