How to use the book spine width calculator
- Pick inches or millimeters and enter the page count. Count both sides of every sheet.
- Enter the paper thickness from your printer and say whether it is for one page or for one sheet (two pages).
- Enter the trim width and height of one page and the bleed your printer asks for.
- Read the spine width and the width and height of the flat cover spread, and use them to set up the cover file.
Formula
Page block thickness = pages x thickness per page, or (pages / 2) x thickness per sheet
Spine width = page block thickness + extra for the wrap
Cover spread width = 2 x trim width + spine width + 2 x bleed
Cover spread height = trim height + 2 x bleed
A cover spread has the back cover, the spine and the front cover side by side, with bleed on the outside edges. The sheets count is half of the page count, rounded up.
A 300 page paperback
A 300 page book on paper 0.0025 in thick per page has a spine of 300 x 0.0025 = 0.75 in. With a 6 by 9 in trim and 0.125 in bleed, the cover spread is 2 x 6 + 0.75 + 2 x 0.125 = 13 in wide and 9 + 0.25 = 9.25 in tall.
A 240 page book on sheets that are 0.005 in thick has 120 sheets and a spine of 0.6 in, giving an 11.85 in wide spread at 5.5 by 8.5 in. In millimeters, 180 pages at 0.1 mm per page make an 18 mm spine, and an A5 book (148 by 210 mm) with 3 mm bleed has a cover spread 320 mm wide.
Where the paper thickness comes from
The thickness of a page depends on the paper weight and type, and it is not the same as the weight. Printers list a value for each paper, often per page or per sheet, and some give a pages per inch figure. Use the printer's value. If you only have a pages per inch figure, divide one by it to get the thickness per page. The paper weight converter can help compare stocks.
For common page sizes see the paper size converter, and the image DPI print size calculator shows what resolution a cover image needs at the spread size.
Always confirm with your printer
Printers have their own templates and may round the spine up or down, add a small allowance for the cover glue or ask for extra bleed. Treat this page as the arithmetic behind the template, and when the printer's template gives a different number, use theirs.
Assumptions and limits
- The thickness per page or per sheet is a figure you enter. The page does not know any paper.
- The spine is the thickness of the pages plus any extra you add for the wrap. Cover boards for hardbacks are not included.
- Bleed is added on all four outside edges of the spread.
- The page count is whole, and the number of sheets is half of it, rounded up.
Frequently asked questions
How do I calculate the spine width of a book?
Multiply the page count by the thickness of one page. 300 pages at 0.0025 inches per page give a 0.75 inch spine.
How wide is the full cover of a book?
Twice the trim width, plus the spine, plus the bleed on both outer edges. A 6 by 9 inch book with a 0.75 inch spine and 0.125 inch bleed needs a 13 inch wide spread.
Is the thickness per page or per sheet?
It can be either, so the page has a choice. A sheet has two pages, so a sheet thickness of 0.005 inches is 0.0025 inches per page.
Why does my printer give a different spine width?
Printers use their own paper data and may round or add an allowance. Use the printer's template when it differs.