When you're artworking a one-piece cover for a book with a flat spine and soft cover (perfectbound, PUR bound or sewn), you'll need to include the right width for the spine. Unless you have a dummy and can measure it with a ruler, or can trust your printer to calculate it for you, this is how to go about it:
If the inner pages are all of the same grade and weight of paper, find out the caliper (thickness) of the paper in microns (thousandths of a millmetre) from the merchant or manufacturer, or measure a sheet with a micrometer. You can't rely on grammage (gsm) because different types of paper bulk up very differently – for instance, a 150gsm 'Volume 15' book paper is 225 microns, but a 150gsm gloss art may be 130 microns. Multiply the caliper by the number of leaves in the book (not the number of pages!) Add up to 1mm to take account of the thickness and foldaround of the cover stock.
If the book contains more than one grade or weight of paper, you'll need to do the calculation for each paper, then add them together.
48 pages (24 leaves) + 4 page cover perfectbound book with text page caliper of 160 micons = 24 x .16 + 1mm
= 4.84mm spine width.
Do get in touch if you have any questions about calculating the spine width of your own book.