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Why spine width matters before you design the cover
The spine of a perfect-bound book is the narrow strip between the front and back covers, and its width decides whether the cover artwork fits. If the spine is too narrow in the design, the cover text wraps onto the front or back. If it is too wide, the spine text floats in empty space and the cover looks wrong. The spine width depends on three things: how many pages the book has, how thick each sheet of paper is, and how thick the cover boards are. Change any one of them and the spine changes.
This calculator works out the spine width from your page count, your paper thickness per sheet, and your cover board thickness. It also shows the sheet count and the text block thickness, so you can see how the number is built up. If you only know the paper weight in GSM, the paper thickness calculator on this site estimates the sheet thickness from weight and density first.
How to use the calculator
- Enter the page count of the book.
- Enter the paper thickness per sheet in millimetres.
- Enter the cover board thickness for each side, or 0 for a soft cover without boards.
- Read the spine width in millimetres and inches, and set your cover file to front cover plus spine plus back cover.
A worked example
Take a 200-page book on 0.1 millimetre paper with 2 millimetre cover boards. Two pages make one sheet, so 200 pages are 100 sheets. The text block is 100 sheets times 0.1 millimetres, which is 10 millimetres thick. Add 2 millimetres of board on each side, and the spine is 14 millimetres wide. The full cover file is therefore the front cover width, plus 14 millimetres of spine, plus the back cover width, plus bleed on the outer edges.
Now double the book to 400 pages on the same paper and boards. The text block doubles to 20 millimetres and the spine becomes 24 millimetres. A cover designed for the 200-page book would be 10 millimetres too narrow on the spine, which is enough to push the spine title off centre. This is why the spine must be recalculated every time the page count or the paper changes, even late in production. A last-minute paper change is the most common cause of a wrong spine.
Perfect binding notes
- Glue adds a small amount beyond the paper stack, so treat the calculated spine as a starting width and confirm it with the printer.
- Coated paper is thinner per GSM than uncoated paper, so the same page count on coated stock gives a narrower spine.
- Odd page counts round up to a full sheet, because a sheet always carries two pages.
- Print a dummy: bind blank pages with the real paper and boards, measure the spine, and check the cover artwork against it.
From spine width to cover file
Once you have the spine width, the cover file is simple arithmetic. Add the front cover width, the spine width and the back cover width, then add bleed on the left and right outer edges. Place the spine text centred on the spine strip, and keep it clear of the fold lines by a millimetre or two, because the fold and the glue area eat into the edges. Check the cover at full size, and if the spine text looks crowded or lost, revisit the paper or the page count rather than squeezing the type. A spine that is measured correctly makes the whole book look professional, and a spine that is guessed makes it look amateur from the shelf.
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