Text on the spine
KDP will not print text on a spine below 79 pages, and what you do print has to fit between the two folds.
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The minimum for spine text
KDP does not print text on the spine of a book with fewer than 79 pages. A spine that thin is only a fraction of an inch wide, and the normal variation in where the cover folds is a large part of that width, so text could not be reliably kept on the spine.
For scale, a 79-page book on white paper has a spine of 0.178".
Fitting text on a spine that allows it
Spine width is page count times paper thickness, so the room you have depends on both. A 200-page book is 0.450" wide on white paper and 0.500" on cream. Keep text well inside both folds; a good rule is to leave clear space on each side at least as wide as the variation you are guarding against.
Run spine text top to bottom, so it reads when the book lies face up — the convention in English-language publishing. Use the title and author surname rather than the full subtitle.
Text that wraps onto the covers
If spine text is too wide for the spine, the excess folds onto the front or back cover. KDP prints this rather than rejecting it, so nothing warns you until the proof arrives with half a letter on the front. The pre-flight checker measures text that crosses a spine fold on an uploaded cover PDF, which is the one place it can be caught before printing.
Designing a spine that reads on a shelf
This is design advice rather than a KDP rule. A spine is seen from a few feet away, edge-on, beside other books. Large, high-contrast type beats decoration: the author's surname and a short title, in a weight that survives being small. A colour band or a simple series mark helps a run of books look like a set.
Keep the spine background continuous with the front and back covers rather than a separate block of colour that has to line up exactly with the folds. If the fold drifts, a continuous background hides it; a hard-edged band shows it immediately as a stripe of the wrong colour on the front cover.
What to do
- Below 79 pages, leave the spine blank; a thin spine cannot be cut accurately enough for text to stay centred.
- Spine width is page count times paper thickness, so cream paper gives a wider spine than white at the same page count.
- Text that crosses the edge of the spine is folded around onto the front or back cover. KDP prints this rather than rejecting it, so nothing warns you before the proof copy arrives — the pre-flight checker measures it on an uploaded cover.
- Binding tolerance shifts the fold slightly either way, so leave a little clearance inside the spine rather than filling it edge to edge.
Calculate spine width, free — no account needed, no upload, the answer in the page.Or calculate spine width in the workspace to save it against a book.