There's no standard for the width of a paperback - it depends on paper thickness and number of pages. The Billy system comes in two widths, 40cm and 80cm. These are the outside dimensions and you'll have to subtract the width of both uprights (i.e. 2x16mm) to get the shelf length. Then, it's a case of measuring the total width of all your books divided by the total of all the shelf lengths to get the answer you need. But remember that there are actually 6 shelves, since the bottom of the unit counts as one.