Why booklet page counts must be a multiple of 4

Fold a sheet of paper in half and you have a little book with four faces: front outside, front inside, back inside, back outside. That is four pages from one sheet, whether it is a mini zine panel or an A4 sheet folded to A5. Nest a second sheet inside the first and you have eight pages; a third makes twelve. There is no way to add a single page, because a page is one face of a sheet that has four.

Canva, Adobe and every print shop will tell you the same thing when you upload a booklet: the page count must be divisible by four. Foldpic's editor shows page counts only in steps of four (8, 12, 16 up to 32) so you never hit the wall at export.

The pages on a sheet are not consecutive. For a booklet of N pages, sheet k has pages N − 2k + 2 and 2k − 1 on the front, and 2k and N − 2k + 1 on the back. For an 8-page booklet that gives sheet 1 with 8 and 1 on the front, 2 and 7 on the back; sheet 2 with 6 and 3 on the front, 4 and 5 on the back. Notice that every pair adds up to N + 1, which is 9 here. That is a check you can do by hand on any imposed PDF: if a pair does not sum to N + 1, something is wrong.

This arrangement is called booklet imposition or saddle-stitch imposition. Foldpic does it for you from the same fold script that draws the instructions; if you ever need to do it in another tool, the formula above is all there is to it.

When your content is not a multiple of four, the booklet is padded up to the next one with blank pages. Where those blanks go is a design choice: at the very back is the usual place, and some tools offer to put the last page on the back cover so the blanks sit inside. Foldpic pads at the back and shows the blanks in the review screen, so you can decide to fill them with a photo, a dedication or a colophon instead of leaving them empty.

For a photo booklet the blanks are rarely a problem. A blank inside back cover looks intentional; a single blank page in the middle does not, so keep the padding at the end.

When sheets are nested and folded, each inner sheet has to wrap around the ones outside it, so its edge pokes out slightly further at the open side. This is creep. The amount is about (number of sheets − 1) times the thickness of the paper. Ordinary 80 gsm paper is about 0.1 mm thick, so a 32-page booklet of eight sheets creeps by around 0.7 mm and a 64-page one by 1.5 mm. That is why the practical limit for a home-stapled booklet is five to eight sheets, 20 to 32 pages.

Creep also shifts the inner pages' content toward the fore-edge relative to the outer pages. Commercial imposition compensates by nudging inner spreads toward the spine; for a home booklet under 32 pages the shift is smaller than the printer's own margin, so the practical fix is simply to keep photos at least 6 mm from the page edge and trim the fore-edge after folding if you want it flush.