Disclosure: Written by Oren Rot (a pen name). I am paid a small fee for placing this article by the maker of BookTemplatesPro, which I mention near the end. The KDP figures below come from Amazon's own help page, linked in the text. The LaTeX preamble is one I compiled myself before posting.
If you typeset a paperback yourself, the margin that trips people up is the inside one. Amazon KDP does not use one fixed gutter. It grows with your page count, because a thicker book curves more into the spine.
The numbers
From KDP's "Set Trim Size, Bleed, and Margins" help page (https://kdp.amazon.com/en_US/help/topic/GVBQ3CMEQW3W2VL6), minimum inside (gutter) margin by page count:
| Pages | Inside margin |
|---|---|
| 24 to 150 | 0.375 in |
| 151 to 300 | 0.5 in |
| 301 to 500 | 0.625 in |
| 501 to 700 | 0.75 in |
| 701 to 828 | 0.875 in |
Top, bottom and outside margins: at least 0.25 in without bleed, 0.375 in with bleed. These are minimums. A book that sits exactly on them tends to look cramped, so most people add a little.
A working preamble
For a 6 x 9 in book of 301 to 500 pages, no bleed:
\documentclass[10pt,twoside]{book}
\usepackage[
paperwidth=6in, paperheight=9in,
inner=0.75in, % KDP minimum is 0.625in at 301-500 pages
outer=0.5in,
top=0.75in, bottom=0.75in
]{geometry}
Two details matter here. First, use twoside. Without it, inner and outer do not swap on even and odd pages and your gutter ends up on the wrong edge half the time. Second, set paperwidth and paperheight to the trim size, not to a bleed size. Bleed belongs to the cover and to full-page images.
Check it before you upload
I compiled the preamble above with showframe on and then ran:
pdfinfo book.pdf | grep -E "Pages|Page size"
The output was Page size: 432 x 648 pts, which is exactly 6 x 9 in (72 points per inch). If you see 612 x 792, you are still on US Letter and KDP will flag the file.
Do this check again after your final edit. Page count drives the gutter. A manuscript that grows from 290 to 310 pages moves from the 0.5 in row to the 0.625 in row, and the margin you set at the start is now too small.
Where a template helps
Everything above is a one-time setup, and it is easy to get right. The harder part is the rest: chapter openings, running heads, front matter, and a cover that matches the interior. If you would rather start from finished pages, BookTemplatesPro sells editable LaTeX interior and cover templates (from $29) with sample PDFs you can inspect first: https://www.booktemplatespro.com/. I have not typeset a book with them. I looked at their public site and sample material only, so judge the samples yourself before buying.
Whatever you use, keep the table above next to your page count.
