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 at the end. The KDP numbers come from Amazon's paperback guidelines, linked below. I ran the script in this post and pasted its real output.
A KDP paperback cover is one flat file: back cover, spine, front cover, and a bleed on every side. The spine is the part people get wrong, because its width depends on your page count and your paper.
The formulas
From KDP's Paperback Submission Guidelines (https://kdp.amazon.com/help/topic/G201857950):
- Spine width, black and white interior, white paper: pages x 0.002252 in
- Spine width, black and white interior, cream paper: pages x 0.0025 in
- Cover width = bleed + back cover width + spine + front cover width + bleed
- Cover height = bleed + trim height + bleed
- Bleed is 0.125 in on all sides
- Keep anything you do not want trimmed at least 0.25 in from the outer edge of the cover
- KDP prints spine text only on books over 79 pages, and wants at least 0.0625 in between the text and the spine edge
A script
def cover(pages, trim_w, trim_h, per_page, bleed=0.125):
spine = pages * per_page
width = bleed * 2 + trim_w * 2 + spine
height = trim_h + bleed * 2
return spine, width, height
for pages, per_page, paper in [(120, 0.002252, "white"), (250, 0.002252, "white"),
(250, 0.0025, "cream"), (320, 0.0025, "cream")]:
s, w, h = cover(pages, 6, 9, per_page)
print(f"{paper:5} {pages:3} pages spine {s:.4f} in cover {w:.4f} x {h:.4f} in")
Output for a 6 x 9 in book:
white 120 pages spine 0.2702 in cover 12.5202 x 9.2500 in
white 250 pages spine 0.5630 in cover 12.8130 x 9.2500 in
cream 250 pages spine 0.6250 in cover 12.8750 x 9.2500 in
cream 320 pages spine 0.8000 in cover 13.0500 x 9.2500 in
What this catches
Wrong paper. The same 250-page book has a 0.563 in spine on white paper and a 0.625 in spine on cream. That is a 0.062 in difference, which is exactly the clearance KDP asks you to leave around spine text. Pick the paper before you design the cover, not after.
A cover that is too narrow. Many cover files are built from the trim size alone, with no bleed. For this book that gives 12.563 in wide instead of 12.813 in. KDP will reject it.
A late page count change. Adding 30 pages on white paper widens the spine by about 0.068 in. If your title text already sits tight on the spine, rerun the script before you upload.
Pixel size at 300 dpi. For the 250-page white-paper book, 12.813 x 9.25 in is about 3844 x 2775 pixels. That is my own arithmetic from the numbers above, so check it against the KDP cover calculator for your exact book.
The script does not replace Amazon's own cover template or calculator. It gives you a fast sanity check while you work, and a record of what you assumed.
If you want the interior done for you
The cover is half the job. The inside needs margins that match your page count, chapter openings and front matter. BookTemplatesPro sells editable LaTeX templates for the interior and cover, from $29, with sample PDFs: https://www.booktemplatespro.com/. I have not built a book with them. I looked only at the public site and samples, so check the samples yourself before you buy.
