Quick answer: For a 300-page paperback at 5.5" × 8.5", the spine width is 0.676" (17.16 mm) on white paper, or 0.750" (19.05 mm) on cream. Spine is 0.676" wide — clears KDP's 0.2" minimum for spine text.
What we're calculating
You're laying out a paperback cover in KDP at the 5.5" × 8.5" trim size, with 300 interior pages. The cover needs three panels — back cover, spine, front cover — plus bleed on every outer edge. The spine width is the only panel that changes with page count and paper choice, and getting it wrong by even 1mm is the most common reason KDP rejects a cover.
Most common trade paperback in the UK. Strong default for novels, memoirs, novellas.
Longer novel or mid-length non-fiction. Premium paperback feel.
The KDP spine-width formula
Spine width = page count × paper-thickness factor
| Paper type | Thickness per page | Used for |
|---|---|---|
| White (60 lb / 90 GSM) | 0.002252″ (0.0572 mm) | Non-fiction, textbooks, illustrated books, anything with colour or images |
| Cream (55 lb / 90 GSM) | 0.002500″ (0.0635 mm) | Novels, memoirs, poetry, literary fiction |
So for your 300-page book:
- White paper: 300 × 0.002252″ = 0.676" (17.16 mm)
- Cream paper: 300 × 0.002500″ = 0.750" (19.05 mm)
That tiny ~10% difference matters — if your designer used white-paper spine width but you tick "cream" at upload (or vice-versa), KDP will reject the cover for spine/trim mismatch.
Full cover-wrap dimensions for 5.5" × 8.5" at 300 pages
KDP wants a single flat cover PDF that includes back cover + spine + front cover, plus 0.125″ of bleed on every outer edge. Here are the exact total dimensions:
| Element | White paper | Cream paper |
|---|---|---|
| Trim height | 8.50″ (215.9 mm) | 8.50″ (215.9 mm) |
| Trim width (per panel) | 5.50″ (139.7 mm) | 5.50″ (139.7 mm) |
| Spine width | 0.676" (17.16 mm) | 0.750" (19.05 mm) |
| Bleed (each outer edge) | 0.125″ (3.18 mm) | 0.125″ (3.18 mm) |
| Total cover-wrap width | 11.926″ (302.91 mm) | 12.000″ (304.80 mm) |
| Total cover-wrap height | 8.750″ (222.25 mm) | 8.750″ (222.25 mm) |
That's the canvas size your designer should set up in Photoshop, Affinity Designer, or Illustrator before placing artwork.
Can you put text on the spine?
| Paper | Spine width | Spine text allowed? |
|---|---|---|
| White | 0.676" | Yes — clears KDP's 0.2" minimum for spine text. |
| Cream | 0.750" | Yes — clears KDP's 0.2" minimum for spine text. |
KDP requires a minimum spine width of 0.2″ (about 79 pages on either stock) before you can place text — title, author name — on the spine. Below that, the spine has to ship blank or with a colour fill only.
Safety guidance
KDP's bindery has a tolerance of roughly ±0.0625″ (1.6 mm) on where the spine actually folds. To stop text wrapping onto the front or back cover:
- Keep all spine text at least 0.0625″ (1.6 mm) from each edge of the spine area.
- Centre text vertically — don't fill the entire spine top-to-bottom.
- For thin spines (under 0.5″), use a condensed typeface and limit to 1 line (title only, no author).
- Always proof in KDP's online 3D previewer before approving.
How to use these numbers
- Open your cover design in Photoshop / Affinity / Illustrator / Canva Pro.
- Set canvas size to 11.926″ × 8.750″ (white) or 12.000″ × 8.750″ (cream).
- Add guides at: 0.125″ (bleed start), 5.625″ (back-cover edge / spine start), 6.301″ (spine end / front-cover start, white paper) — or 6.375″ for cream.
- Place spine text in the spine zone with the 0.0625″ safety margin on each side.
- Export as flattened PDF/X-1a, no transparency, no crop marks, fonts embedded.
- Upload to KDP and check the 3D preview before approving.
Other page counts at 5.5" × 8.5"
- 250-page 5.5" × 8.5" spine width — 0.563" white / 0.625" cream
- 400-page 5.5" × 8.5" spine width — 0.901" white / 1.000" cream
Same page count (300) at other KDP trim sizes
- 300-page 5" × 8"
- 300-page 5.06" × 7.81"
- 300-page 5.25" × 8"
- 300-page 6" × 9"
- 300-page 6.14" × 9.21"
- 300-page 6.69" × 9.61"
- 300-page 7" × 10"
- 300-page 7.5" × 9.25"
- 300-page 8" × 10"
- 300-page 8.5" × 8.5"
- 300-page 8.5" × 11"
Pre-flight your cover
Spine-width mismatches are the #1 cause of KDP cover rejections we see at publishing.co.uk. Our free KDP Readiness Score checks your interior page count, recalculates the spine width for your paper type, and warns if the uploaded cover's spine doesn't match — plus 35+ other rules KDP enforces. Sixty seconds, no signup gate.
Related guides
- KDP Spine Width Calculator (interactive) — change paper and page count, see spine update live
- KDP error: spine too narrow for spine text — what to do if KDP rejects your cover for spine text
- KDP Trim Size Guide — picking the right trim for your genre
- KDP Margin Calculator — gutter margins scale with page count too
- White vs Cream Paper — which to pick and why it changes spine width
- KDP Cover Formatting Guide — full cover spec with bleed and safe zones
Frequently asked questions
What is the spine width for a 300-page 5.5" × 8.5" KDP book?
On white paper: 0.676" (17.16 mm). On cream paper: 0.750" (19.05 mm). The trim size (5.5" × 8.5") doesn't affect spine width — only page count and paper type do — but it does change the full cover-wrap height and panel widths.
Why is the cream-paper spine wider than white at the same page count?
Cream stock is physically thicker (0.0025″ vs 0.002252″ per sheet), so 300 pages of cream produces a 0.074" (1.89 mm) thicker spine than the same 300 pages on white.
Does my cover need to be the cream-paper size if I tick cream at upload?
Yes. The spine width on your cover PDF must match the paper you choose at KDP upload, to the third decimal. If you submit a white-paper-sized spine but choose cream stock at upload (or vice-versa), KDP will reject the cover. Pick paper first, design after.
Can KDP's cover template generator do this for me?
Yes — and you should use it as the final check. Enter trim 5.5" × 8.5", page count 300, and your paper type at KDP's cover template generator. It outputs a PDF with all guides at the exact dimensions. The values above match what that tool will give you.
What if my page count changes after the cover is designed?
Re-run the calculation. Even a 10-page change shifts the spine by roughly 0.025″ — enough to push text out of safe zones. If the interior changes, regenerate the cover.
About this guide
One of 96 programmatic spine-width references on publishing.co.uk — twelve KDP trim sizes × eight page-count buckets, covering the combinations we see most often in book-formatting jobs. Written by Robert Prime, founder of publishing.co.uk and co-runner of the LoveReading network, with 500+ KDP submissions overseen.
If you'd rather skip the math entirely, our formatting service is from £69 — KDP-ready interior PDF plus cover spec sheet with exact spine width, in 3 days.
