Everything about a printed page is a choice except one thing. The inside margin — the strip of white between your text and the spine — is not a design decision. It is a physical allowance for the fact that a bound book does not open flat, and getting it wrong is the single most common reason a first proof copy looks amateur.
The margin that is not a margin
Open any paperback on your desk and look at where the pages disappear into the binding. The paper curves away from you. Text near that curve is harder to read, and on a thick book some of it is genuinely difficult to see without cracking the spine.
So the inside margin has to be wider than the outside one. Not for balance — because part of it is swallowed by the shape of the book. The extra width is called the gutter, and it is why a page that looks perfectly centred as a PDF looks shoved toward the spine once printed.
Why it changes with page count
A thicker book curves more. That is the whole reason.
Amazon KDP publishes a minimum inside margin as a function of page count, and it is a requirement rather than a suggestion — a file below it is rejected at upload rather than flagged:
| Page count | Minimum inside margin |
|---|---|
| Up to 150 | 0.375″ |
| 151–300 | 0.5″ |
| 301–500 | 0.625″ |
| 501–700 | 0.75″ |
| 701–828 | 0.875″ |
Past 828 pages KDP will not bind a single paperback at all, which usually means two volumes.
The outside, top and bottom margins have a floor too — 0.25″ for a no-bleed interior — but that is genuinely a minimum rather than a target. At 0.25″ a novel looks cramped and your thumb covers the text. Something nearer 0.5″ outside and 0.75″ top and bottom reads like a book.
The circular part nobody warns you about
Here is where it gets awkward, and it is not obvious until you are in the middle of it.
The margin depends on the page count. The page count depends on the margin.
Widen the gutter and the text block narrows. A narrower text block fits fewer words per page. Fewer words per page means more pages. More pages can push you into the next band — where the gutter widens again.
For most books this settles after one step. A 290-page manuscript that grows to 310 has crossed from a 0.5″ gutter to 0.625″, gains a few more pages, and then stops. But you have to actually check, because the version you laid out is no longer the version you are printing.
The way out is to lay the book out once to learn its true length, then lay it out again with the geometry that length requires. That is exactly what our print book formatter does — it renders the whole interior to count the pages, then re-renders if the count moved you into a different band. If you are doing it by hand in Word or InDesign, do the same thing manually: set a provisional gutter, generate the book, read the final page count, and go back and fix the margin.
Mirrored margins
The second thing that catches people. The inside margin is on the left of a right-hand page and the right of a left-hand page, because the spine alternates sides as you turn the pages.
If you set a single fixed left margin for the whole document — which is what happens by default in most word processors — then every other page has its extra width on the wrong side. Text drifts toward the spine on the versos and away from it on the rectos. It is subtle enough that you might not name it when you see it, and obvious enough that the book looks wrong.
In Word this is the Mirror margins setting under Page Setup, where "left" and "right" become "inside" and "outside". If you have never turned it on, your interior is not mirrored.
Working out your own numbers
If you want the figures without laying anything out, our margin and gutter calculator takes a trim size and a page count and gives you the inside margin KDP requires, the resulting text block, and a warning when you are within twenty-five pages of the next threshold. It also estimates a page count from a word count, for the common case of not having laid the book out yet.
For the cover rather than the interior, the spine calculator does the other half — spine width from page count and paper stock, which is a different calculation and a different failure.
Order a proof
Correct margins are necessary and not sufficient. A proof copy costs a few dollars and is the only way to see how the type actually sits on paper: whether the gutter is genuinely comfortable, whether your chosen size reads well at arm's length, whether the paper shows through. Every printed book you admire went through at least one.
Do that once before ordering a hundred.