in Production
Why spine width depends on page count and paper
The spine is the one part of a cover whose size you cannot choose. It follows from the page count and the paper, and it has to be right.
A book's front and back covers are fixed by its trim size. The spine is different. Its width is not a design choice but a result: the combined thickness of every sheet in the book, plus an allowance for the cover and binding. Get it wrong and the spine text drifts onto the front or back, or the cover sits loosely around the book.
Pages, leaves and caliper
A leaf is a single sheet of paper in the book, printed on both sides. Each leaf carries two pages, so a 300-page book contains 150 leaves.
The thickness of a single sheet is called its caliper, usually given in microns or thousandths of an inch. Some printers express the same information as pages per inch: the number of pages, not leaves, that stack up to one inch. This is not the same as the pixels per inch used for image resolution, although both are sometimes abbreviated to PPI.
The basic calculation is simple. Multiply the number of leaves by the caliper, or divide the page count by the pages-per-inch figure, and you have the thickness of the book block. As a hypothetical example, if a paper's caliper were 0.1 mm, 150 leaves would come to 15 mm. Printers then add an allowance for the cover, which differs between paperbacks and hardbacks.
Why the paper matters as much as the page count
Two books with the same page count can have very different spine widths. Papers vary in thickness, and a bulky uncoated paper can be noticeably thicker than a smooth coated paper of similar weight. Cream and white stocks from the same supplier may differ as well.
Paper weight, measured in grams per square metre, is not the same as thickness. A heavier paper is usually thicker, but a high-bulk paper can be thicker than a heavier, denser one. Only the caliper tells you the thickness.
Why it matters in production
A paperback cover is printed as a single flat sheet: back, spine and front, with bleed around the outer edges. The spine panel has to match the finished book closely. If it is too narrow, spine text creeps onto the front or back. If it is too wide, the edge of the front or back design wraps round onto the spine.
Because there is always some variation in binding, designers keep spine text a little inside the spine edges and avoid placing strong lines or sharp colour changes exactly on the fold, where a small shift would be obvious.
Hardbacks add a further step. The case is made from boards covered in cloth or printed paper, and its spine width has to allow for the board thickness and the construction. Printers calculate this for their own process.
The practical sequence
The page count has to be final before the spine width can be. Any change that adds or removes pages, such as a new chapter, a different type size or an added index, changes the spine. The paper stock has to be settled too.
The reliable approach is to ask the printer, or use their cover template or spine calculator, with the final page count and the exact paper stock. Print-on-demand services generally produce a cover template for a given trim size, page count and paper. Offset printers will supply the spine width on request, and can often make a blank dummy from the actual paper so the bulk can be checked in the hand.
Only then should the cover be finalised. A cover designed before the page count is fixed is a draft, however finished it looks.