Operational headroom
Will the collection still fit when checked-out items return?
Compare normal on-shelf occupancy with a full-return scenario. Any annual-growth estimate is shown separately and never treated as a promise.
Enter local counts and measured usable run.
Keep returns separate from new growth
Currently borrowed items already belong to the collection. The full-return scenario adds them back before testing your chosen working capacity. Net annual additions are a separate assumption; enter zero when you do not want a growth horizon.
This is an aggregate linear-space estimate, not proof that every item fits an individual shelf. A thick book cannot use small leftover gaps on different shelves. Use the mixed-format allocation planner to test individual shelf boundaries, height and depth.