What is the Calculation based on? There’s a simple formula which determines that width. Just take the number of pages in your book and divide that by your text paper’s PPI (Pages Per Inch). PPI depends on what kind of paper you’re using. Let’s say your book has 200 pages and you are printing it on a paper which has a PPI of 400. Then the width of your book’s spine will be 200 ÷ 400 or half an inch. That’s for a paperback. For a hard cover book, you have to add the thickness of the boards.