Deterministic space management queries are answered in a deduplication context. A block subkey data structure comprises a subkey (a subset of the bits of a corresponding hash key identifying a block) and refcount (an indication of the total number of objects that reference a given block) for each block in a deduplication store. For each object, an object subkey data structure is maintained. An object subkey data structure comprises a list of subkeys corresponding to blocks referenced by that object. To answer a space management query concerning a given object set, a corresponding object set subkey data structure is created, comprising an entry for each block referenced by at least one object in the object set. Each entry comprises a subkey identifying that block and an object set refcount for that block. The data structures are utilized to efficiently answer space management queries.