The gutter is the inside margin, nearest the spine, and the only margin whose minimum changes with the page count. Amazon sets it by page count, because a thicker book loses more of each page into the binding. The difficulty is that the margin you look up changes the page count you looked it up with.
The table
| Page count | Minimum inside margin |
|---|
The outside, top and bottom margins need at least 0.25″ without bleed, or 0.375″ with bleed. The page count does not change them.
The circular part
A manuscript has no page count until it is laid out, and the page count depends on how much text fits on a page, which depends on the margins. A wider gutter fits less text, which means more pages, which can cross into the next band and require a wider gutter still.
laid out at 0.375″ → comes to 158 pages
158 pages → needs 0.5″
laid out at 0.5″ → still 158 pages → settled
Usually one adjustment like this is enough, but only if the book is laid out again and the real page count read back each time. Laid out once at 0.375″ and uploaded at 158 pages, the gutter is below the minimum for its own length. KDP's rejection then names the margin, not the cause.
Near a band edge
A book that lands near the edge of a band, at 150 or 152 pages, may need a second pass. A wider gutter fits less text on a page, so it does not make the book shorter, and the adjusting ends; it may end in the wider band.
Near the edge, take the wider gutter and stop. It is legal on both sides of the edge, and the narrower one is legal on only one.
Rule of thumb. When two gutters seem to apply, the wider one is always valid. It costs a fraction of an inch of line length.
The minimum is a floor, not a target
Amazon's numbers mark the point below which a book cannot be bound and trimmed reliably. A book set exactly there meets the requirement, but looks cramped. Books that read well sit above it, with margins that are not symmetrical:
- The bottom margin is deeper than the top, so the text sits slightly high. A block centered by measurement looks too low.
- The outside margin is generous, because the reader's thumb goes there.
- The gutter is wider than the minimum, so the text does not run into the curve of the binding.
For a 6 × 9 book at 280 pages, good margins are 0.875″ inside, 1″ outside, 0.6875″ top and 0.8125″ bottom, with the text in EB Garamond at 11.5 points. The lines then come to about 70 characters, inside the 55–75 that reads best. With margins close to Amazon's floor, lines at 11 points run past 80. Publishers set a wider page in larger type and keep the line about the same length.
What to do
- Lay the book out with a first guess at the gutter.
- Read the real page count from the file, not an estimate from the word count.
- Look up the gutter that page count requires.
- If it differs from the one you used, lay out again and repeat.
- If it flips between two values, take the wider one and stop.
- Check that the final page count is even. KDP rounds an odd count up to the next even number, so add the blank page yourself, and before the cover is drawn: the page widens the spine.
The margin calculator handles steps three and five for a page count you already have. Torcul runs the whole loop: it lays out, reads the count back and adjusts until they agree, taking the wider gutter between two bands.
None of this guarantees that a file will be accepted. It is what the published requirements say as of ; check the date on any report a tool gives you.