Publishing Intelligence.
Tools for self-publishing authors — spine width, royalty per copy, trim sizes, page count and ISBN checks. Figures follow Amazon KDP’s published specifications, which change periodically.
Royalty Estimator
Instantly estimate your net earnings across US, UK, India, and European marketplaces. Includes marketplace fees and variable printing costs.
Launch ToolSpine Expert
Calculate exact spine width for white and cream stocks. Uses Amazon KDP’s published PPI (Pages Per Inch) constants for each paper stock.
Launch ToolISBN Check Digit
Generate the check digit for a 12-digit ISBN prefix, or validate an existing 13-digit ISBN before submitting metadata.
Launch ToolNew to self-publishing?
Every term explained — PPI, bleed, trim size, royalties, and more.
Standard Trim Sizes
The Order These Decisions Have to Be Made In
Self-publishing calculations are not independent. They chain, and taking them out of order is the most common reason an author ends up rebuilding a cover a week before launch. The sequence below is the one the print process actually imposes.
1. Trim size comes first
Trim size determines how many words fit on a page, which determines your page count, which determines both your printing cost and your spine width. Choosing it late means every downstream number changes. Genre convention matters here too: 6×9 for most non-fiction and trade fiction, 5×8 or 5.25×8 for literary fiction and memoir, 8.5×11 for workbooks and technical titles.
2. Page count follows from trim size and formatting
A 70,000-word manuscript is roughly 233 pages at 300 words per page on 6×9, but closer to 280 at 250 words per page on a pocket trim. Front and back matter add another 10 to 20 pages. This number drives the two that follow, so it is worth estimating before you commission anything.
3. Printing cost sets your price floor
Print-on-demand charges a fixed amount per book plus a per-page rate, so cost scales with page count rather than word count. That fixed cost is why very short books are disproportionately expensive to print, and why a 400-page paperback can struggle to work at a competitive retail price. Your printing cost, not your ambition, sets the minimum viable list price.
4. Royalty is what is left
Amazon KDP pays 60% of list price minus printing cost on paperbacks. The subtraction happens after the percentage, which catches people out: on a $12.99 book with a $4.20 printing cost, you receive $7.79 minus $4.20, so $3.59 — not 60% of $12.99. Expanded distribution drops the rate to 40%, which often turns a slim margin negative.
5. Spine width is calculated last
Spine width depends on final page count and paper stock, so it cannot be fixed until the interior is finished. Cream and white paper have different thicknesses, which means the same book produces two different covers. Below roughly 80 pages the spine is too narrow to carry text at all, and KDP will silently drop spine content rather than reject the file — so the first you learn of it is when the printed proof arrives blank.
Research & Notes.
In-depth articles on the math and logic behind our tools — written for people who want to understand the calculations, not just run them.
How to Calculate KDP Spine Width Without Getting Your Cover Rejected
A spine width miscalculation is the single most common reason KDP rejects a cover file. Here is the exact formula, the constants, and the edge cases that trip up most authors.
KDP vs IngramSpark: Which Pays Better Royalties in 2026?
Both KDP and IngramSpark promise broad distribution, but their royalty structures are fundamentally different. Here is an honest comparison of which platform pays more and when.
Why Your Book Spine Goes Blank: The 80-Page Rule Explained
KDP will not print text on spines below a page count threshold. Here is the physics behind why, and what you can do about it if your manuscript is short.