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Book Spine Calculator
Use this free book spine calculator to find the spine width of a book before you design its cover. Enter the page count, pick the paper type, choose the cover stock, and the calculator gives you the spine width in inches and millimetres, plus the total width of the cover spread you need to send to the printer.
Count every printed page, including blanks. A 200-page novel is a typical example.
Ask your printer for the PPI of your exact paper, or measure a stack: PPI = pages in the stack ÷ stack height in inches.
The hinge allowance is the extra width for the two folds where the cover bends, usually 0.25 inches in total.
| Amount | Value |
|---|---|
| Paper thickness (pages ÷ PPI) | 0.50 in |
| Cover stock (both covers) | 0.04 in |
| Spine width | 0.54 in |
| Spine width in millimetres | 13.7 mm |
| Two front covers | 12.00 in |
| Hinge allowance | 0.25 in |
| Total cover width | 12.79 in |
How it works
Every book cover is built around one number: the spine width. Get it wrong and the title sits off-centre, the back cover text runs onto the spine, or the printer rejects the file. This calculator works out the spine from the two things that decide it — how many pages the book has, and how thick each sheet of paper is.
Paper thickness is usually given as PPI, or pages per inch: the number of pages that stack up to one inch. A thin 70 gsm uncoated paper packs 440 pages into an inch, while a thick 150 gsm coated paper manages only 240. Divide the page count by the PPI and you get the thickness of the page block in inches. A 200-page book on 80 gsm paper at 400 PPI has a page block of 200 ÷ 400 = 0.50 inches.
Spine width = page block thickness + 2 × cover thickness
Total cover width = 2 × front cover width + spine width + hinge allowance
The cover stock adds a little more. A paperback cover is about 0.02 inches thick, and there are two of them — front and back — so they add 0.04 inches. A hardcover board is much thicker, about 0.125 inches per side, which adds 0.25 inches to the spine. That is why a hardcover of the same book always has a wider spine than its paperback twin.
The total cover width is the number your cover designer actually needs. It is the front cover, plus the spine, plus the back cover, plus the hinge allowance — the small extra width, usually 0.25 inches in total, where the cover folds at the spine. With a 6-inch front cover, a 0.54-inch spine, and a 0.25-inch hinge allowance, the full cover spread is 6 + 0.54 + 6 + 0.25 = 12.79 inches wide.
If your printer gave you a PPI value, choose the custom option and type it in — that is always the most accurate route. If you only know the paper weight in gsm, the presets are good estimates for common uncoated and coated stocks. When in doubt, ask the printer: a spine that is a fraction of an inch off is visible on the shelf, and printers would rather confirm the number than reprint a cover.
Worked example
You are designing the cover for a 200-page paperback novel. The book is printed on 80 gsm uncoated paper, the front cover is 6 inches wide, and you allow 0.25 inches for the hinges.
Step 1: Find the page block thickness — 80 gsm uncoated paper is 400 PPI, so 200 ÷ 400 = 0.50 inches.
Step 2: Add the cover stock — a paperback cover is 0.02 inches thick on each side, so 2 × 0.02 = 0.04 inches.
Step 3: Add them for the spine — 0.50 + 0.04 = 0.54 inches, which is 0.54 × 25.4 = 13.7 millimetres.
Step 4: Work out the total cover width — 6 + 0.54 + 6 + 0.25 = 12.79 inches.
The result is a 0.54-inch (13.7 mm) spine and a 12.79-inch total cover width — exactly what the calculator shows for these inputs.
FAQ
What is PPI in book printing?
PPI means pages per inch: how many pages of a particular paper stack up to one inch thick. Thin paper has a high PPI, around 440 for 70 gsm uncoated, and thick coated paper has a low PPI, around 240 for 150 gsm. Divide your page count by the PPI to get the thickness of the page block, which is the main part of the spine width.
Do I count blank pages in the page count?
Yes. Count every page in the finished book, including blank pages, title pages, and end matter. The spine depends on the physical stack of paper, not on how much text is printed on it. If your book has 200 numbered pages plus 4 unnumbered pages, enter 204.
Why is a hardcover spine wider than a paperback spine?
Because the boards are thicker. A paperback cover adds about 0.04 inches in total, while hardcover boards at 0.125 inches each add 0.25 inches. For the same 200-page book on the same paper, the hardcover spine is about 0.21 inches wider than the paperback spine.
What is the hinge allowance for?
The hinge is the fold where the cover bends at the edge of the spine, on both the front and the back. The allowance, usually 0.25 inches in total, gives the cover room to open without cracking the spine. It is added to the total cover width, not to the spine width itself.
What if my printer uses millimetres?
The calculator shows the spine in both inches and millimetres, so you can give the printer either number. A 0.54-inch spine is 13.7 millimetres. Most printers work in millimetres, so send the millimetre figure and keep the inches as a cross-check.