What it means
Spine width is the thickness of the block of pages, and therefore the width of the cover panel that shows on the shelf. It is not chosen; it is calculated: page count × the paper's thickness per page (its caliper), sometimes with a small allowance for the cover boards of a hardcover.
Printers publish the figures. Typical uncoated white book paper runs about 0.0022–0.0025 in per page, cream stock slightly more, coated colour stock about 0.0023 in — so a 300-page paperback on cream paper has a spine close to 0.75 in (19 mm), and a 100-page one about 0.25 in.
The number matters because the cover is printed as one wrap — back, spine, front — and a spine drawn even 1 mm too wide or narrow lets the spine text creep onto the front or back. Most print-on-demand services allow spine text only above roughly 80 pages; thinner books get a blank spine.
In Bindery
Bindery sizes the spine from the real page count of the typeset book, then validates the cover file against the printer’s rules before anything prints.
Make the book, not the checklist
Describe a book or upload a manuscript. Bindery writes with you, typesets in LaTeX, sizes the spine from the real page count and produces printer-validated files — the watermarked proof is always free.
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