What percent of the library's total collection is checked out at any given time? Is it on a scale of millions? Someone said if all the books were recalled at once there wouldn't be enough space to sustain it all...
Putting aside space considerations for a moment, if all books checked out were recalled the howls of anguish from distressed patrons would be enough to make the Suggestions Office close up shop and head for the hills.
But let's assume everyone voluntarily returns their books at once. We have around 180,000 books checked out at any given time. Compared to our total holdings (7.9 million volumes at last count), this is miniscule: only 2.3% of the collection is off the shelf!
However, the tide of books would easily overwhelm our shelving operations, which normally process 180,000 books over 8-10 weeks. And there's no shelf space to store that many books as they are checked in and sorted.
But let's answer the real question here: does the Library leave space on the shelves for checked-out books, even though we haven't seen some of them in 20 years? Although we try to not fill the shelves more than 85% full -- to account both for returned books and new acquisitions -- recent space constraints have forced us to far exceed these limits. We'd have to shift almost the entire collection to accommodate all the returned items, and even then we might not be able to fit everything on the shelf.
That's one of the reasons we're building a new library.