Spine width calculator
A spine is half your page count, times the thickness of one sheet — plus, if a print-on-demand service is binding it, whatever their own formula says instead. Type your page count below and the answer updates as you type. No file, no upload, no email: it is pure arithmetic, and it is the same model this site uses to size the covers it sells.
Pages, not sheets — a sheet carries two pages.
A shop can caliper the exact stock for you — ask, then trust their number over any calculator.
- Full cover, flat
- 311 mm wide (front + spine + back; add 3.2 mm bleed each side if the printer asks)
- Lettering on the spine
- Yes — it fits
Nothing to upload — this is arithmetic, and it runs right here. The model behind it is the same one that sizes the covers this site sells, so the calculator cannot drift from the bindery.
The arithmetic, so you can check it
A sheet carries two pages, one each side, so a 300-page book is 150 sheets. On standard 80 gsm bond — 0.10 mm a sheet — that block is 150 × 0.10 = 15.0 mm of spine. Every paper the tool offers states its per-sheet thickness; a shop can measure the exact stock with a caliper in ten seconds, and their number beats any calculator, including this one. (Weights and their US basis-name chaos: the paper converter untangles lb and gsm exactly.)
Two honest models, one answer each
| Who is binding | The model | What to watch |
|---|---|---|
| A local print shop | Caliper: pages ÷ 2 × the real thickness of the chosen sheet. | Ask the shop to caliper the stock. Do not guess paper weights — 20 gsm of wrong is 2 mm of spine on a long book. |
| Lulu (print on demand) | Their published formula — pages ÷ 444 + 0.06 inches — plus a board table for hardcovers. | The flat 1.5 mm is the cover wrap; leave it out and every cover you build is narrow. Their hardcover spines come from a table, not a formula, because board comes in steps. |
| Other POD services | Each publishes its own caliper and wrap. | KDP, IngramSpark and BookBaby each state theirs. Use their own calculator for their own product — this tool models a shop and Lulu and says so. |
Spine text has floors, not just width
Lettering needs 6.4 mm (a quarter inch) of spine to survive binding drift — and print-on-demand services add their own floor on top (Lulu's is 80 pages, whichever gives way first). Below that, put the title on the cover and leave the spine clean; text that rolls around onto the cover looks like exactly the mistake it is. The tool tells you when you clear both floors — and the lettering fitter goes further, sizing your actual title for the spine you have.
Questions people actually ask
Pages or sheets — which do I count?
Pages. One sheet carries two pages, and the calculator does the halving. If your manuscript is in Word, the page count it reports is the number to type — then re-check once it is a PDF, because exported PDFs often repaginate.
Does this work for KDP?
Not directly. Amazon KDP publishes its own per-page calipers (different for black-and-white, standard colour and premium colour paper), and their calculator applies them for you. This tool models a local shop's caliper and Lulu's published formulas; for KDP use KDP's own, because a cover built on the wrong caliper is rejected at upload.
Why does Lulu's spine come out wider than my shop's arithmetic?
Their formula adds a flat 1.5 mm for the cover stock wrapping the block, on top of their paper caliper. A local shop's binder wraps differently again. Neither is wrong — they are different physical books, and the calculator keeps the two models apart for exactly that reason.
What's the full cover width for?
A wraparound cover is front + spine + back as one flat sheet, so its width is twice the trim width plus the spine. If your printer wants bleed, add 3.2 mm (an eighth of an inch) each side — the tool's number excludes it so you add it deliberately, not by accident. When the flat cover is built, the crop-marks tool puts trim marks and bleed on the finished file.
My book is very long. Is there a spine too wide?
Yes. Past roughly 25 mm a glued paperback stops lying open and starts cracking at the gutter. The usual answer is volumes — splitting the book into a set — which also happens to make a better object. (Long books are heavy books, too: the book weight calculator itemises what a 600-pager carries.)
Wondering how many pages your chat will be? The studio counts them free.
Count my pages — freeThis is one of the tools — or see everything we’ve written.