A cover file that is a fraction of an inch off will be rejected or print with the spine text on the wrong edge. The sizes follow simple formulas, so you can calculate them once and brief your designer exactly.
Step one: the spine width
The spine is as thick as your pages. Multiply the page count by the paper thickness: 0.002252 inches per page for white paper, 0.0025 for cream. Premium colour paper is slightly thicker, at 0.002347 inches per page. Use the final page count, including blank pages, because a different count changes the spine.
Step two: the full cover size
The cover is one flat image: back cover, spine and front cover, plus bleed on every outside edge. Bleed is the extra 0.125 inches that gets trimmed off.
- Width = (trim width x 2) + spine width + 0.25 inches
- Height = trim height + 0.25 inches
Worked example
A 6 x 9 inch book with 300 pages on white paper:
- Spine = 300 x 0.002252 = 0.6756 inches
- Width = 12 + 0.6756 + 0.25 = 12.9256 inches
- Height = 9 + 0.25 = 9.25 inches
At 300 pixels per inch the file is about 3878 x 2775 pixels.
Spine text and the barcode area
KDP's cover creator only allows spine text on books of a minimum page count, and thin spines are hard to read at any size. If you design your own cover, keep spine text well inside the spine so a small shift in printing does not clip it. Leave the barcode area on the back cover clear, because KDP places the barcode there.
What to send your designer
Send the trim size, page count, paper colour, spine width and total cover size in inches and pixels. The cover and spine calculator produces all of these in one place. Ask for a print-ready PDF and a flat image for the ebook cover.
If the page count changes
Every added or removed page changes the spine. Ask your designer to wait for the final page count, or pay for a revision. Our spine width article covers the edge cases.