KDP Trim Sizes Explained: Which Size to Pick for Your Book in 2026

Trim size is one of the first decisions you make when uploading to KDP, and it's also one of the most over-thought. There are really only four trim sizes that matter for 95% of authors. Pick the right one and your book looks professional; pick the wrong one and readers feel it before they can articulate why.

5 × 8 — Standard for genre fiction

The classic mass-market paperback feel. Use for novels, novellas, and short story collections. Holds 200–400 pages comfortably and looks "right" on a fiction shelf. Royalties are highest at this size because it uses the least paper.

6 × 9 — The default for non-fiction

Non-fiction, business books, self-help, biographies, and longer literary novels. Slightly more authoritative than 5×8 and gives margins room to breathe. If you're not sure which size to pick for non-fiction, pick this one.

8.5 × 8.5 — Children's picture books

The square format buyers expect for illustrated children's books ages 3–8. Pairs perfectly with two-page spreads. KDP supports it natively for both paperback and hardcover.

8.5 × 11 — Workbooks, journals, activity books

US-letter size. The right pick for puzzle books, planners, coloring books, and any book where the user writes inside it. Avoid for novels — it looks like a textbook.

Bleed, gutter, and margin rules that prevent rejections

• Bleed: add 0.125″ on the outer three edges if your interior has art or color extending to the page edge. Skip bleed for text-only interiors.
• Gutter: KDP requires 0.375″ minimum gutter for books up to 150 pages, scaling up to 0.875″ for 700+ pages.
• Outer margins: never go below 0.25″ — readers' thumbs hate it and so does the printer.

The decision rule

Genre fiction → 5×8. Non-fiction → 6×9. Picture book → 8.5×8.5. Anything the buyer writes in → 8.5×11. Don't overthink it. The wrong trim size won't kill your book, but the right one quietly signals "this author knows what they're doing."