Your cover is a single flat sheet: back cover, spine and front cover side by side, with bleed all around. To build it, you need one number nobody can guess — the spine width — and that depends on how many pages you have and how thick your paper is.

The basic formula

For a paperback, the spine is simply the thickness of the text block:

spine width = page count × thickness per page

Printers express paper thickness in one of two ways. Print-on-demand services such as KDP publish a per-page factor: 0.002252 in for white paper, 0.0025 in for cream, 0.002252 in for standard colour and 0.002347 in for premium colour. Offset printers usually quote PPI (pages per inch), so the formula becomes:

spine width = page count ÷ PPI

A 300-page book on white paper is therefore 300 × 0.002252 = 0.6756 in (17.16 mm). On cream it is 0.75 in — cream book paper is bulkier. See every combination in the spine width charts.

From spine to full cover

Once you know the spine, the full cover dimensions follow:

  • Width = bleed + back cover width + spine + front cover width + bleed
  • Height = bleed + trim height + bleed

With the common 0.125 in bleed, a 300-page 6 x 9 in paperback needs a cover of 12.926 x 9.25 in. The calculator draws this layout and lets you download a true-size SVG template.

Why your printer's template wins

Formulas are perfect for planning, but each printer's actual paper varies slightly by batch and mill. IngramSpark, for example, generates a cover template from your ISBN, trim and page count, and that template is the one to use for final files. If you print with several services, build your cover with layers so you can move the front and back panels when the spine changes.

Hardcover spines are different

A case-bound book's spine includes the boards and a hinge allowance, so it is wider than the text block — and the cover wraps around the boards with a much larger margin. Never reuse a paperback cover for a hardcover; always start from the printer's hardcover template.

Designing a spine that survives printing

Binding machines fold covers within a tolerance of about 1/16 in (1.6 mm) in either direction. Design with that in mind:

  • Keep spine text at least 0.0625 in from each fold line.
  • Avoid hard colour changes exactly at the fold: let the spine colour or image extend slightly onto the covers, or use a continuous background.
  • Do not put thin borders or frames around the spine — a small shift makes them visibly uneven.
  • On thin books (under about 80–100 pages), leave the spine blank or use only a colour; text will not fit and may not be allowed.
  • In the US and UK, spine text usually reads top to bottom; much of continental Europe reads bottom to top. Follow your market's convention.

What else affects thickness

Coated papers are thinner per page than uncoated papers of the same weight; heavier papers are thicker; and humidity can make a stack swell slightly. For offset jobs, ask the printer for a bulking dummy — a blank book made from your exact paper — and measure it.