Book margin & gutter calculator

KDP's inside margin requirements

These are minimums for a no-bleed interior, and they are checked at upload rather than treated as advice.

The circular part

The margin depends on the page count, and the page count depends on the margin. Widening the gutter narrows the text block, which adds pages, which can move you into the next band — where the gutter widens again.

For most books it settles after one step, which is why the calculator flags it when you are within twenty-five pages of a threshold. Our print formatter handles it by rendering the whole interior once to count the pages, then re-rendering with the geometry that count actually requires.

Questions

What is a gutter?

The inside margin — the one against the spine. It has to be wider than the outside margin because a bound book curves away from you near the binding, swallowing part of the page. The thicker the book, the more it swallows.

Why does the required margin change with page count?

Because the spine gets thicker. KDP publishes a table: 0.375″ up to 150 pages, then 0.5″, 0.625″, 0.75″ and 0.875″ as the book grows. These are requirements — a file under them is rejected at upload, not flagged as a suggestion.

There's a chicken-and-egg problem here, isn't there?

Yes, and it's real. A wider inside margin narrows the text block, which adds pages, which can push you into the next band and widen the margin again. That is why the calculator warns when you are close to a threshold. Our print formatter resolves it by rendering once to count pages, then re-rendering with the correct geometry.

Are the outside, top and bottom margins fixed?

No. KDP's floor for a no-bleed interior is 0.25″ on those three, but 0.25″ looks cramped in a finished book. The figures shown are what we use for a comfortable novel page: 0.5″ outside, 0.75″ top and bottom.

Do I need to add bleed?

Only if something is meant to run off the edge of the page — a full-bleed image or a coloured background. A text interior does not, and bleed changes the trim dimensions, so leave it off unless you know you need it.

Is this the same maths your formatter uses?

The same functions, literally — the calculator and the print formatter import from one module. If they disagreed, one of them would be wrong about a file you were about to send to a printer.