Spine width, bleed and full-wrap covers: the print math
Print-on-demand rejects more covers than interiors, and nearly always for the same three reasons. Here is the arithmetic, so you can check a file before you upload it.
A print cover is one file, not three
Back cover, spine and front cover are a single PDF, laid out left to right, at the printer's computed dimensions. A front-only file is an ebook cover. Uploading one to a print service is the most common first-time rejection there is.
Spine width
Spine width comes from page count multiplied by a paper factor, and then — the part most templates omit — a flat allowance for the cover stock itself.
| Element | Value |
|---|---|
| White paper factor | 0.002252 in per page |
| Cover stock allowance | +0.06 in, flat |
| Example: 300 pages | (300 × 0.002252) + 0.06 = 0.7356 in |
Leave off the 0.06 and the file is roughly a sixteenth of an inch narrow. That is the error that comes back as expected cover size X, submitted Y in the print previewer, and it is why a cover that "looks right" fails preflight.
Bleed
Add 0.125 in of bleed on all four sides of the full wrap, and let background art run into it. Trim moves by up to an eighth of an inch on a POD line. Anything you care about — type, faces, rules — stays at least 0.25 in inside the trim line.
The barcode zone
Keep roughly 2 × 1.2 in clear in the lower right of the back cover. The retailer prints a barcode there whether or not your design has room for it, and it will land on your closing paragraph if you put one there.
Spine text
Below about 100 pages, leave the spine blank. There is not enough width to set type without the risk of it wrapping onto the front or back panel once trim drift is accounted for.
The order of operations
- Lock the interior. Final page count.
- Compute spine width from that page count.
- Get the printer's computed cover dimensions and build to those exact numbers.
- Export as a single flattened PDF with fonts embedded and no transparency.
- Read the previewer output rather than trusting your own render.
Change the page count after step two and you start again at step two. That is why back matter belongs in the manuscript before typesetting.