What is the gutter in a book?
The gutter is the inside margin of a printed page, the side that runs into the binding. It has to widen as a book gets thicker, because more of the page curves out of sight. KDP sets a minimum for each page count, from 0.375 in up to 0.875 in.
What it means in practice
KDP’s minimum inside margin, by the book’s page count:
| Pages | Minimum inside margin |
|---|---|
| 24–150 | 0.375 in |
| 151–300 | 0.5 in |
| 301–500 | 0.625 in |
| 501–700 | 0.75 in |
| 701–828 | 0.875 in |
The gutter swaps sides from page to page: it is on the left of a right-hand page and on the right of a left-hand one. So the left and right margins of a printed page differ, and they swap on the next page.
How Chapbook uses it
A paperback interior adds KDP’s minimum for its page count to the outside margin, so the inside margin is always the wider one. Widening the gutter can change the page count, so Chapbook lays the book out again until the two agree. Its paperback stops at the end of the third band in the table.
The preflight report under the file lists its margins beside KDP’s minimums. The sample on this page is a digital PDF with no binding, so its left and right margins match.
In a real book
Related terms
- What is trim size? Trim size is a printed book's final width and height, after the pages are cut. The sizes KDP offers, what they change, and the ones Chapbook's paperback uses.
- What is bleed in book printing? Bleed is artwork that runs past the cut line, so it reaches the page edge after trimming. KDP's bleed size, the page it makes, and when Chapbook adds it.
- What is a book spine? The spine is the bound edge of a book, the part you see on a shelf. How KDP's paper and page count set its width, and when it can carry text.
Make one
Questions people ask
Is the gutter the same as the inside margin?
At KDP, yes. Its help page, checked on 30 October 2026, says every page has one inside margin, also called the gutter.
Why does the gutter depend on the page count?
A thicker book doesn't open as flat, so more of each page disappears toward the spine. KDP's minimum rises in steps as the page count grows.
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