Work out spine width and full cover size for a paperback
This book spine width calculator turns your page count and paper type into the spine thickness, then adds the front and back panels and the bleed to give the exact size of a full wrap cover. Print-on-demand services such as Amazon KDP need a single cover image that is the right width for the book, and a spine that is even slightly wrong can shift the text or fail the file check.
Pick the paper from the list (the factors are the ones Amazon KDP publishes), choose the trim size, and read the spine width in inches and millimetres, the cover width and height with bleed, and the pixel size at 300 dpi. Everything is computed in your browser.
How to use the book spine width calculator
- Enter the page count of your final interior file. Print books use an even number, so an odd count is rounded up.
- Choose the paper and ink. Use Custom if your printer gives a different thickness per page.
- Pick the trim size or type your own, in inches or millimetres.
- Read the spine width and the full cover size, and use the layout drawing to set up your cover template.
Book Spine Width Calculator features
KDP paper factors
White, cream, standard colour, premium colour and groundwood thicknesses, plus a custom value.
Full cover size
Back, spine and front with bleed, in inches or millimetres and in pixels at 300 dpi.
Layout drawing
A to-scale picture of the back, spine and front with the bleed edge.
The spine width formula
Spine width is the page count multiplied by the thickness of one page of the chosen paper. Amazon KDP’s help pages (read 24 October 2026) give these factors in inches per page: 0.002252 for black-and-white on white paper, 0.0025 on cream paper, 0.002252 for standard colour, 0.002347 for premium colour and 0.00235 for groundwood. A 300-page book on white paper is 300 × 0.002252 = 0.6756 inches, about 17.2 mm; on cream it is 0.75 inches.
The factor is per page, and two pages make one sheet, so it is about half the thickness of a single sheet. Other printers use their own paper, so use the Custom option with the figure from their template if you are not printing through KDP. The result is an estimate: printers allow a small tolerance for paper and binding.
Full wrap cover dimensions
A full cover is the back panel, the spine and the front panel side by side, with a bleed of 0.125 inches (3.2 mm) added on the top, bottom and outer edges so that colour runs past the cut line. Width = 2 × trim width + spine + 2 × bleed; height = trim height + 2 × bleed. A 6 × 9 inch book of 300 white-paper pages therefore needs a cover of 12.926 × 9.25 inches, which is 3,878 × 2,775 pixels at 300 dpi.
Keep important text well inside the trim edge and away from the spine folds, and check the printer’s own template, which also shows barcode and safe-area rules. Hardcover books need extra allowance for board and hinge that this calculator does not include.
Common mistakes to avoid
Take the page count from the finished, exported PDF, not from your word processor, because page numbering and front matter change it. Choose the paper before designing the cover: a cover made for white paper and printed on cream will have a spine that is too narrow. Very thin books, usually under about 100 pages, have spines too narrow for readable text, so the title is better kept on the front. Finally, re-run the calculation whenever you add or remove a page, since every extra pair of pages widens the spine by a few thousandths of an inch.
Book Spine Width Calculator FAQ
How do I calculate spine width?
Multiply the page count by the paper thickness per page, for example 300 × 0.002252 = 0.6756 inches for KDP white paper.
How thick is a 300-page book spine?
About 0.68 inches (17 mm) on white paper and 0.75 inches (19 mm) on cream paper.
Why is my page count rounded up?
Pages are printed on both sides of a sheet, so a book has an even number of pages. An odd count is rounded up by one.
What is bleed?
Extra artwork past the trim line so the cut does not leave a white edge. KDP asks for 0.125 inches.
Does this work for hardcovers?
The spine figure applies to the page block only. Hardcover cases need the printer’s template.
Which size should the cover image be?
Use the full cover size shown, and the resolution your printer requests, normally 300 dpi.