Why spine width matters
When you design a paperback cover, the back cover, spine and front cover are printed as one sheet. The spine width depends on the number of pages and the paper, so you need it before you can lay out the cover file.
The formula
Spine width = page count × paper thickness per page
Print-on-demand services publish their per-page thickness. Amazon KDP, for example, uses:
| Paper | Thickness per page |
|---|---|
| White | 0.002252 in |
| Cream | 0.0025 in |
| Premium color | 0.002347 in |
Each page is one side of a leaf: a 300-page book has 150 sheets of paper.
Full cover size
Cover width = bleed + back cover + spine + front cover + bleed
Cover height = bleed + trim height + bleed
Bleed is the extra image beyond the trim line (usually 0.125 in) so that no white edge shows after cutting.
Example
A 300-page, 6 × 9 in book on white paper has a spine of 300 × 0.002252 = 0.676 in (17.2 mm). The full cover with 0.125 in bleed measures 0.125 + 6 + 0.676 + 6 + 0.125 = 12.93 in wide by 9.25 in high.
Design tips
- Most printers do not allow text on the spine below about 80 to 100 pages, because the spine is too narrow to print reliably.
- Keep spine text at least 0.0625 in (1.6 mm) from each fold.
- Keep important text and images at least 0.25 in inside the trim on the front and back.
- Always download the printer’s own cover template once you know the final page count: paper stock and binding glue vary slightly.
Offset printing
For offset printing, the printer calculates the spine from the paper’s caliper or “PPI” (pages per inch). A 50 lb uncoated stock at about 500 PPI gives 300 pages a spine of 0.6 in, plus the cover board thickness for hardcovers.