The quantification of the impacts of green spaces on health has been attracting increasing attention in political discussions and research activities. The results of several research projects are now converging towards a quantifiable association of the effects of green spaces on health in various populations and on a variety of spatial and temporal scales. Current knowledge on green space measurements and effects adds a public health perspective that is relevant for monitoring progress towards the Sustainable Development Goals (in particular Goal 11.7). At the end of 2016, the WHO European Centre for Environment and Health (ECEH) in Bonn, Germany, started working on a prototype tool, GreenUr, for the quantification of the impacts of green spaces at urban scale. ECEH based the tool on discussions with experts and on various projects it has carried out over the past years, such as its review of the evidence of the health benefits of urban green space. GreenUr is a plug-in for QGIS, a free and open-source desktop geographic information system (GIS). GreenUr offers the possibility to measure availability and accessibility of green space in cities. It accommodates algorithms to calculate potential direct health effects of green space, for example, on mental health, and indirect impacts, for example, through ecosystem services. The tool can serve as an educational, communication and scientific support. It is intended for use by local authorities, urban planners, nongovernmental organizations, students and researchers. The tool is currently under development and testing, and the first release is expected by 2020.