The spine width formula
Spine width = page count × paper thickness per page. The thickness is fixed by the paper the printer uses. These are the figures KDP publishes, and the same ones this calculator applies:
| Paper | Per page | 100 pp | 200 pp | 300 pp | 400 pp | 500 pp |
|---|---|---|---|---|---|---|
| Cream paper | 0.0025" | 0.250" | 0.500" | 0.750" | 1.000" | 1.250" |
| White paper | 0.002252" | 0.225" | 0.450" | 0.676" | 0.901" | 1.126" |
| Premium colour | 0.002347" | 0.235" | 0.469" | 0.704" | 0.939" | 1.173" |
Worked example
A 300-page novel printed on cream paper: 300 × 0.0025" = 0.750", or 19.0 mm. Printed on white paper the same book is 0.676" thick. If your interior has 301 pages, the printer adds a blank page and the spine is calculated on 302.
Why the spine width matters
The spine sits between the back and front covers on a single full-wrap cover file. If the spine on your file is narrower or wider than the printed book, the title wraps onto the front or back cover and KDP may reject the upload. The page count must therefore be final before the cover is finished — any edit that adds or removes pages changes the spine.
Common mistakes
- Using the manuscript page count from Word. Word pages at Letter size are not book pages. Count pages in the trim-sized interior PDF, or estimate with the page count calculator.
- Designing for white and printing on cream. Pick the paper first; it changes the spine.
- Putting text on a spine under 80 pages. KDP will not accept it, because the spine is too narrow to print text safely.
- Filling the spine edge to edge with text. Leave a small margin on each side — printing and folding shift slightly from copy to copy.
Next, get the full cover dimensions with bleed from the book cover size calculator.