Urbanisation is altering local flora and fauna, but urban greenspaces can provide refugia for a variety of taxa. However, we often lack basic biodiversity information (e.g., species richness) for these urban greenspaces. Citizen science projects are continuously improving our understanding of ecology at broad temporal and spatial scales. But, many conservation-relevant decisions are idiosyncratic and made at small management scales (e.g., local government). Given a general bias of citizen science data towards areas with large human populations, citizen scientists are best placed to contribute to improving our understanding of the biodiversity within cities and urban greenspaces. We introduce the Greenspace Bird Calculator: a web-app aimed at enhancing our collective knowledge of bird diversity in urban greenspaces. Users of the web-app could be land managers seeking to understand the bird diversity in the greenspaces they manage. It is built in a reproducible workflow, allowing anyone to delineate a greenspace and submit it to the web-app administrator, receiving an output comprising the greenspace’s total bird diversity. The Greenspace Bird Calculator web-app provides an automated tool to utilise existing eBird citizen science data to calculate species richness for urban greenspaces globally. Critically, the GBC web-app statistically assesses available data that otherwise would be unlikely to be considered by decision-makers. This web-app is an example of the evolution of citizen science, whereby the data collected has been analysed to allow accessible interpretation and inclusion into urban greenspace management and planning.