A saddle-stitch booklet is folded sheets, stapled through the fold. Each folded sheet makes four pages — two on each side — so a booklet's page count must be a multiple of four. If yours is not, blank or filler pages are added at the back. This page tells you how many sheets you need, and exactly which pages share each sheet.
Sheets are listed outermost first — sheet 1 is the sheet you see from the outside of the finished booklet. Each sheet has two sides: the outside (the two pages printed on one side of the sheet) and the inside (the two pages on the other side). When you fold the sheet, the left and right pages land in the order shown. Page numbers above your count are blank pages, added at the back to reach a multiple of four.
Worked example: a 20-page booklet needs 5 sheets. Sheet 1's outside is pages 20 and 1, its inside is pages 2 and 19. Sheet 5, the innermost sheet, is pages 12 and 9 outside, 10 and 11 inside. An 18-page booklet also needs 5 sheets, with pages 19 and 20 blank.
Planning aid only. It does not upload files, print, or impose a PDF — use the table to lay out your own pages in your design software, and check your printer's own imposition or booklet setting before a full run. Thick booklets may need creep (page shift) adjustment, which this page does not calculate; ask your printer about it for booklets over about 40 pages.