Bleed, margins & gutters: print-ready PDF settings explained

Most "my book got rejected" problems come down to four words: bleed, crop marks, margins and gutter. They sound technical, but each one is simple once you see what it's protecting against — a printer's blade is never perfectly accurate, and these settings give it room to be slightly off without ruining your book. Here's what each means and the numbers you actually need.

Bleed

Bleed is extra image area that extends past the trim line, so that anything meant to run to the very edge of the page still does after the blade trims it. The industry standard is 0.125 inches (3 mm) on every side. You only need bleed if you have full-page or edge-to-edge images — a cover, a photo, a background. A plain text interior needs no bleed at all. If you turn bleed on, your PDF pages become slightly larger than the trim size (trim + 0.125" each side), which is exactly what the printer expects.

Crop marks

Crop marks are little corner lines that show the printer where to cut. Here's the catch most people miss: print-on-demand services don't want them. KDP and IngramSpark expect a PDF sized to trim-plus-bleed with no marks — they trim automatically. A local offset print shop, on the other hand, often does want crop marks on a larger sheet. So the rule is: marks off for KDP/IngramSpark, marks on for a traditional printer who asks for them.

Safe margins

The "safe area" is the opposite of bleed — it's how far your text should stay inside the trim so nothing important gets cut off or sits awkwardly at the edge. Keep body text at least 0.25 inches from the trimmed edge on the outside, top and bottom. Most templates handle this for you, but it's worth checking if you've set very tight margins.

The binding gutter (the one people forget)

The gutter is the inside margin — the edge nearest the spine. On a perfect-bound paperback, part of that inside edge curves into the binding and is hard to read, so the gutter has to grow with the page count: a thicker book needs a wider gutter. Amazon KDP's guidance looks like this:

Page count	Minimum inside (gutter) margin
Up to 150	0.375 in
151–300	0.5 in
301–500	0.625 in
501–700	0.75 in
701–828	0.875 in

Set the gutter too small for a thick book and the first or last words of every line disappear into the spine — a classic reason proofs come back wrong.

One more: RGB vs CMYK

Computer screens are RGB; traditional offset presses think in CMYK. The good news for indie authors: print-on-demand services (KDP, IngramSpark, Lulu) accept RGB PDFs and convert them for you, so RGB is fine for the vast majority of books. Only a traditional offset print shop is likely to ask for CMYK, and they'll usually convert it if you ask.

How Bookmint handles all of this

When you export a print-ready PDF in Bookmint, a quick pre-flight appears: you pick your printer (Amazon KDP, IngramSpark or a local shop) and it sets crop marks correctly for you, adds the 0.125" bleed when you have full-page images, and checks your binding gutter against your page count — with a one-click fix if it's too small. It also flags any images that are below 300 dpi at print size, so soft pictures don't surprise you in the proof. The trim size and embedded fonts are baked in, so the file is ready to upload or hand to a printer.