Spine width from page count and the real bulk of the stock, without the additive term that makes most print-on-demand calculators 13% too wide.
Grammage does not tell you how thick the book will be
Every spine calculator I could find takes a page count and either a stock name or a gsm figure. Neither determines thickness. Three text stocks at 118 gsm — coated gloss, coated matte and uncoated offset — run 526, 471 and 333 pages per inch respectively.
On a three-hundred-page book that is a spine of 14.5 mm, 16.2 mm or 22.9 mm. Same weight, same page count, eight and a half millimetres apart. A cover designed against the wrong one will have its spine text sitting on the front board.
The figure that matters is bulk, published by the mill as pages per inch or as a caliper. This calculator takes it directly, which as far as I can tell no other one does.
The +0.06 that made the search results wrong
KDP publishes a per-page multiplier and nothing else: 0.002252 inches per page for white paper, 0.0025 for cream, 0.002347 for premium colour. There is no additive term.
Several widely-used KDP spine calculators add 0.06 inches anyway, which makes a two-hundred-page white-paper spine 0.5104 in instead of 0.4504 — thirteen per cent too wide. I think I found where it came from. Lulu publishes its formula as page count divided by 444, plus 0.06 inches. And one divided by 444 is 0.002252 exactly — precisely KDP's white-paper multiplier.
So the two formulas share a number, someone spliced them, and Lulu's cover allowance has been travelling around attached to KDP's multiplier ever since. It is now the consensus answer in search results. This calculator applies each service's own published formula and shows you what the wrong one would have given.
Pages per inch, not sheets per inch
A leaf of paper is two pages. Printers publish bulk as pages per inch; paper mills publish caliper per leaf. The two differ by a factor of exactly two, and they are confused constantly.
If a spine figure comes out at half or double what you expected, this is almost always why. The formula is simply pages divided by pages-per-inch — or equivalently, half the page count times the caliper of one leaf.
Hardcover is a different question
KDP publishes no hardcover multiplier at all. The figure circulating as a "hardcover" number is its premium-colour paperback multiplier relabelled. A hardcover spine also has to account for the case board, which on KDP is two millimetres thick, and for the wrap and hinge allowances.
Lulu handles hardcover as a lookup table rather than a formula, which is a fair admission that it does not reduce to one. IngramSpark publishes no spine formula at all, for either binding. For a hardcover, use the service's own cover template generator and treat any formula you find online with suspicion.
Asked often
What is the KDP spine formula?
Page count times the paper multiplier — 0.002252 in for white, 0.0025 for cream, 0.002347 for premium colour. There is no additive term, despite what many calculators do.
Why do calculators give different answers?
Most add 0.06 inches, which is Lulu's cover allowance rather than anything KDP publishes. It makes a 200-page spine about 13% too wide.
Can I work out the spine from gsm?
No. Grammage is mass, not thickness. Three 118 gsm stocks give an 8.4 mm spread on a 300-page book. You need the bulk or caliper figure.