Green spaces play an important role in urban areas. We study the accessibility of green urban areas by combining open data sets about green with population size data. We develop a mathematical model to define the population density of a green area and calculate the available green space depending on the location. To this end, we do not only consider walking distance to and size of the green area, but also take into account the local population size. Our model quantifies how the available green space depends on the location in the city, such that heavily populated areas have a small amount of green available, even when closely located to a green area.