Two numbers and a page count
The text block is the page count divided by the pages-per-inch figure of the paper, and that is the whole of the interesting arithmetic. Two hundred and forty-eight pages at 400 ppi is 0.620 inches of paper. Everything after that is what the binder adds: on a perfect bound book, the glue and roughing at the spine plus the cover stock on both faces, which on the defaults is 0.030 plus 0.024, giving a 0.674 inch spine — eleven sixteenths, or 17.1 mm.
Pages per inch and caliper are the same thing written differently. A sheet of paper carries two pages, so pages per inch is 2 divided by the caliper of one sheet: 400 ppi is a 0.005 inch sheet. If your printer quotes one and your layout software wants the other, that division is the entire conversion and it is worth doing rather than looking up a chart.
The cover is one artboard
A perfect bound cover prints flat and gets scored twice. The artwork is back panel, spine, front panel, plus bleed all round: 6 plus 0.674 plus 6 plus a quarter inch of bleed comes to 12.924 by 9.25 inches for a 6 by 9 book. The two score lines sit half the spine either side of the centre, which is 0.337 inches from the middle of the sheet. Anything that has to line up across the spine — a photograph running over, a rule, a background colour change — is keyed off those two lines.
A case bound cover is a different shape again, because the case wraps boards. Each panel is the page plus the square and the turn-in, there is a hinge gap on each side of the spine, and the spine itself has to clear both boards. The page works it out, but the numbers all belong to your binder rather than to a standard, which is why every one of them is a field.
Where this goes wrong
Bulk is not held to a tight tolerance. Give the default book a bulk figure that is 10 percent out either way and the spine lands anywhere between 0.612 and 0.736 inches — a range of 0.124, which is two sixteenths of an inch. On a cover with a dark front and a white back, that much error puts a visible band of the front colour round onto the back panel, or leaves a white sliver on the spine. It is the single most common cover error and no calculator can fix it.
What fixes it is a dummy. Ask the printer to bind an unprinted block on the actual stock at the actual page count, measure it with a caliper, and lay the cover out on the measured number. Everything on this page is an estimate to work from until that dummy exists.
Mixed stocks and plate sections
A book with a plate section is two bulk calculations added together. Sixteen pages of 250 ppi coated stock in the middle of a 248-page book at 400 ppi adds 0.064 inches, which is another sixteenth on the spine on its own. The same applies to a heavier endpaper, a tipped-in card, or a text stock that changes halfway through a reprint because the first one went out of production. Add each stock at its own bulk rather than averaging them.
Questions people ask
How do I calculate the spine width of a book?
Divide the page count by the pages-per-inch figure for the paper, then add what the binding needs. A 248-page book on 400 ppi stock has a 0.620 inch text block; a perfect bound edition adds 0.030 for glue and 0.012 twice for the cover wrapping both faces of the spine, giving 0.674 inches. Case bound adds endpapers, both boards and a spine allowance instead.
What is pages per inch and where do I get it?
It is how many pages of that stock stack into an inch, and it comes from the mill datasheet or from your printer. It is a property of the paper alone — 400 ppi means each sheet is 0.005 inches thick, since a sheet carries two pages. Never take it from a similar paper at the same weight, because bulk and weight are independent specifications and two stocks at the same weight can bulk quite differently.
How wrong can a spine width be?
Enough to matter. Bulk figures are not tight, and 10 percent either way on a 248-page book moves the spine across a range of about an eighth of an inch. That is plainly visible as the front colour wrapping onto the back panel. The reliable fix is a bound dummy on the real stock at the real page count, measured with a caliper before the covers are printed.
What size should the cover artwork be?
For a perfect bound book it is two page widths plus the spine plus bleed all round. A 6 by 9 book with a 0.674 inch spine and an eighth of bleed needs a 12.924 by 9.25 inch artboard, with score lines 0.337 inches either side of the centre. A case bound wrap is bigger again because it has to clear the boards, the hinge gaps and the turn-in.
Does the ink or the lamination change the spine?
Not by anything you can measure on a normal book. Heavy solid coverage on a coated stock can bulk a block very slightly and a laminate adds a fraction of a thousandth to the cover, both of which vanish inside the tolerance on the paper bulk itself. Spend the effort on getting a dummy bound rather than on modelling the ink film.