Choose on page count first, then check availability, then look at the object. Word count determines how many pages a given trim produces, page count determines whether the spine works and what the book costs to print, and availability determines whether the size closes off a hardcover or a distribution channel you will want later.
Decide early. Trim size sets the text block, so changing it reflows the entire book.
Step One: Start From Word Count
A smaller page holds fewer words, so it produces more pages from the same manuscript. That is the whole mechanism, and it drives everything else.
| Manuscript | Look at |
|---|---|
| Under 60,000 words | 5 x 8 |
| 60,000 to 100,000 | 5.5 x 8.5 or 6 x 9 |
| 100,000 to 120,000 | 6 x 9 |
| Over 120,000 | 6 x 9, and check the page ceiling |
These are starting points, not rules. The judgement they encode is that a book should feel substantial without becoming a brick.
Step Two: Check the Spine
This is the check people skip, and it is the one that produces a visible failure.
Spine width is page count multiplied by paper thickness. KDP publishes the multipliers: 0.002252 inches per page on white paper, 0.0025 on cream. A 200 page book on white paper has a spine of about 0.45 inches. A 120 page book has about 0.27 inches, which is too narrow for comfortable spine text.
KDP prints no spine text at all on books with 79 pages or fewer. IngramSpark's threshold is 48 pages for paperbacks and 18 for hardcovers, following a change dated 23 April 2026. A book below the threshold has a blank spine on the shelf, which is a real commercial problem in a bookshop and irrelevant online.
If your page count leaves the spine too thin, the fix is a smaller trim size, which raises the page count. See how to calculate spine width.
Step Three: Check Availability Before You Commit
The smaller sizes close off options, and the pattern is not obvious from any platform's size list:
| Size | KDP hardcover | IngramSpark hardcover | IS premium colour | KDP Expanded Distribution |
|---|---|---|---|---|
| 5 x 8 | No | All three types | Yes | Yes, except premium colour |
| 5.25 x 8 | No | None | No | Yes, except premium colour |
| 5.5 x 8.5 | Yes | All three types | Yes | Yes, all papers |
| 6 x 9 | Yes | All three types | Yes | Yes, all papers |
| A5 | No | Case laminate only | No | Not listed |
Three consequences worth knowing. 5.25 by 8 has no hardcover at either platform, which no other common size manages. A5 is not a KDP paperback size in the US store at all, appearing only through Amazon's Japanese storefront, though it falls inside KDP's custom trim range. And KDP hardcovers are excluded from Expanded Distribution entirely, at every size, so a hardcover that needs bookshop and library distribution has to come from IngramSpark.
If a hardcover is possible later, 5.5 by 8.5 and 6 by 9 keep every route open.
Step Four: Look at the Object
Once the arithmetic and availability are settled, the remaining question is aesthetic and it is legitimate.
6 by 9 reads as a standard trade paperback, which is exactly what most readers expect and therefore invisible. 5 by 8 reads as more considered and is common in literary publishing. A5 reads as European. The proportions differ as much as the areas do: A5 is taller and narrower than 5.5 by 8.5 despite being larger.
The genre convention is worth a glance rather than obedience. Look at four or five books your reader already owns, and check whether yours sitting beside them looks like it belongs.
What Does Not Matter As Much As You Think
Popularity. 6 by 9 is Amazon's stated most common size for US books and its setup default, but no source publishes trim size data for novels specifically. Choosing by page count beats choosing by what is common. See most common paperback size for novels.
Printing cost in isolation. 5 by 8, 5.5 by 8.5 and 6 by 9 are all in KDP's regular trim class, so the size itself does not change the rate. What changes cost is the resulting page count, which moves in the opposite direction to the page size. A large trim size at KDP is "either more than 6.12 inches (155.5 mm) in width or more than 9 inches (228.6 mm) in height", and those cost more, but none of the common novel sizes crosses that line.
What Deckle Does
Deckle offers six trim sizes: 5 by 8, 5.5 by 8.5, 6 by 9 (the default), 6.14 by 9.21, BoD 135 by 215 mm and A5 148 by 210 mm. There is no 5.25 by 8, which given its hardcover restrictions is rarely a loss.
Two features make this decision testable rather than theoretical.
Estimated page count is shown before you export, so you can compare sizes on the number that actually drives spine width, gutter requirement and cost. Change the trim, read the new estimate, and you have your answer.
Gutter enforcement is automatic. Deckle takes the larger of your inside margin and the KDP minimum for the estimated page count, so moving to a smaller trim, which raises the page count, cannot silently leave you below the required gutter. If you are printing through IngramSpark, add 0.125 inches to your inside margin for their bind strip, which Deckle does not add.
The exported print PDF carries bleed on three edges regardless of size, so a 6 by 9 book comes out at 6.125 by 9.25, matching both platforms' specifications.
For what changes downstream once you pick, see book trim sizes explained and book margins and gutter explained.
Common Mistakes
- Choosing last. Trim size sets the text block, so it reflows the whole book.
- Choosing by popularity. Page count is the variable that determines whether the object works.
- Ignoring the spine. Under 80 pages at KDP there is no spine text at all.
- Picking 5.25 by 8 without checking hardcover. Neither platform prints one.
- Picking A5 for a US release. It is not a listed KDP size in the US store.
- Assuming a KDP hardcover reaches bookshops. It never does, at any size.
FAQ
Q: What is the safest choice if I am undecided? A: 6 by 9 or 5.5 by 8.5. Both have full availability in every binding, paper and distribution channel.
Q: Can I change trim size later? A: Technically yes, but it reflows the entire book and changes the page count, spine and margins.
Q: Does a smaller book cost less to print? A: Not necessarily. It produces more pages, and page count is what drives cost.
Q: How do I know if my spine is too thin? A: Multiply page count by 0.002252 for white paper. Below about 0.3 inches, spine text becomes difficult.
Q: Should I match my genre's usual size? A: It is worth checking, but page count and availability are the constraints that can actually go wrong.