If you publish a paperback through Amazon KDP, your PDF is rejected when the inside margin, the gutter, is too narrow for the book. How narrow is too narrow depends on how many pages the book has:
| Pages | Minimum inside margin |
|---|---|
| 24–150 | 0.375″ |
| 151–300 | 0.5″ |
| 301–500 | 0.625″ |
| 501–700 | 0.75″ |
| 701–828 | 0.875″ |
It looks like a lookup table. It isn't one, because a manuscript has no page count until it has been laid out, and the page count depends on the margins. A wider gutter fits less text on each line, so the book gets longer, and a longer book can need a wider gutter still.
A worked example
Say a novel is expected to run to 150 pages. 150 pages need 0.375″, so it is laid out at 0.375″, and it comes to 158 pages. 158 pages need 0.5″. Laid out again at 0.5″, it is still 158 pages, and the gutter and the page count finally agree.
Lay it out once at 0.375″ and upload the 158-page PDF, and the gutter is below the minimum for the book's own length. KDP's rejection then names the margin, not the cause, and the author goes looking for the problem in the wrong place.
Why the loop ends
Each pass can only move the gutter one way. A wider gutter fits less on a page, so it does not make a book shorter, and the required gutter never gets smaller as the page count grows. The gutter can only increase and there are five bands, so there are at most five passes. In practice one adjustment is usually enough.
The awkward case is the edge of a band. Real layout is not perfectly smooth: widow and orphan control, and chapters that must start on a right-hand page, move whole pages at a time. So the loop has an exit for a book that flips back and forth across a band boundary. When that happens, the wider gutter wins, because it is valid on both sides of the boundary and the narrower one is valid on only one. No manuscript in Torcul's test corpus has ever taken that exit. It is there so the loop can never spin.
That exit still had a lesson in it. Every other way out of the loop pads an odd page count to an even one. KDP rounds an odd count up by adding a blank page of its own, and the spine would then be sized for a book one page shorter than the one being printed. The exit for an oscillating book was the only one that didn't pad. Since no manuscript ever takes it, nothing noticed for a while.
The cover depends on it too
The spine is as wide as the stack of paper, so its width comes from the page count and the paper. The full cover, front, spine and back, can only be sized once the interior has settled. A cover made from an estimated page count is a cover for a slightly different book.
What Torcul does about it
Torcul sets the book with Typst, reads the real page count back from the PDF, chooses the gutter for that count and sets the book again until the two agree. It pads the count to even, sizes the cover from the final number, and then checks the finished PDF, margins, widows and orphans, before anything is downloaded. It can be tried on a sample book at torcul.com/app.
If you format by hand
- Decide the gutter after you know the page count, not before.
- Check it again after anything that moves text: the font size, the trim size, the front matter, a new chapter.
- Near a band edge, take the wider gutter. It is valid on both sides.
The bands, the outside margins and how to check a file are in How KDP gutter margins work.