📐 Sizes, spines and printers

A print cover is one wrap — back, spine and front on a single sheet — measured outward from your trim size. Here is where every number comes from, and what goes wrong when one of them is off.

Spine width is page count times paper bulk, and every printer's paper bulks differently. The same 300-page novel has a different spine at each of them, which is why one formula copied off a blog won't do.

Trim size is the finished page size of your book, and it is the first decision you make — because page count, spine width, margins, cover proportions and the cost of every copy are all derived from it.

Four print-on-demand routes, what each one is actually for, and the technical fact that catches most authors out — each printer measures a book differently, so one cover file is not valid everywhere.

The three bindings are three different physical objects, not three finishes of one file. A hardcover wrap is bigger than its trim size, its spine is thicker, and a jacket adds two flaps that need copy.

Almost every rejected print file fails for one of ten reasons, and the same ten come up at every printer. Here they are, most common first, with what the printer sees and how to fix it.

Two ideas hold every print file together — artwork that runs past the edge, and anything you care about held well inside it. Learn them properly once and most rejections stop happening.