Provided are a computer program product, system, and method for generating a configuration page for rendering in a graphical user interface (GUI) for managing provisioning of storage resources in a storage system. A determination is made of allocation commitment certainties for virtual storage ranges to allocate from the storage system based on physical capacity in the storage system and an estimated data reduction rate. Each of the allocation commitment certainties indicates a likelihood that a corresponding of the virtual storage ranges will be available for allocation in the storage system. Visual representation of a plurality of user selectable storage spaces indicating the allocation commitment certainties for the virtual storage ranges in the user selectable storage spaces are rendered in the configuration page. A user selection graphic that when selected provisions a selected of the user selectable storage spaces in the storage system is rendered in the configuration page.