How KDP calculates spine width
A paperback spine is simply the stack of paper inside it. KDP publishes a thickness per page for each paper and multiplies it by your page count. Page counts are always even: KDP prints on both sides of each sheet, so an odd count is rounded up to the next even number before the spine is calculated.
| Ink & paper | Per page | 100 pages | 300 pages |
|---|---|---|---|
| Premium color · white paper | 0.002347 in · 0.0596 mm | 0.235 in | 0.704 in |
| Standard color · white paper | 0.002252 in · 0.0572 mm | 0.225 in | 0.676 in |
| Black & white · white paper | 0.002252 in · 0.0572 mm | 0.225 in | 0.676 in |
| Black & white · cream paper | 0.0025 in · 0.0635 mm | 0.25 in | 0.75 in |
| Black & white · groundwood paper | 0.00235 in · 0.0597 mm | 0.235 in | 0.705 in |
The same factors are used by KDP’s own cover calculator. Our numbers are checked against it; the only difference is that we always use the even page count KDP prints, while the public calculator uses the number exactly as typed.
Hardcover spines
KDP hardcovers are case laminate: the cover is wrapped around rigid boards. The spine is the paper thickness plus 4.8 mm for the boards and the binding. Hardcovers need between 75 and 550 pages and are only offered on premium color · white paper, black & white · white paper, black & white · cream paper.
When can the spine carry text?
KDP only allows spine text on books with at least 79 pages, and asks for 0.0625 in of clear space between the text and each fold, because folds can move by up to 0.0625 in during binding. On a thin spine, those margins leave almost no room for type. kdpbook therefore only sets spine text when the safe area can hold 7 pt type, KDP’s minimum text size:
- Premium color · white paper: spine text from 96 pages
- Standard color · white paper: spine text from 100 pages
- Black & white · white paper: spine text from 100 pages
- Black & white · cream paper: spine text from 90 pages
- Black & white · groundwood paper: spine text from 96 pages
Mistakes that get covers rejected
- Making the cover before the interior is final. Add 20 pages and the spine grows by about 0.05 in on cream paper.
- Choosing a different paper in KDP than the one used for the cover. Switching from white to cream widens the spine by about 11%.
- Putting text on the spine of a book under 79 pages, or too close to the folds.
Next step: turn the spine width into a full cover with the cover size calculator, which also gives you a template to design on.