A press prints on a large sheet, not on book pages. Imposition is the arrangement that decides where each page sits on that sheet so that after printing, folding and trimming, the pages end up in the right order and the right way up. Get it wrong and the finished booklet reads 1, 4, 5, 8, 3, 2.
Folding dictates the page count. Each fold doubles the number of page panels on a sheet, so a sheet folded once gives 4 pages, twice gives 8, three times gives 16. That is why:
Pages per sheet is a straightforward area calculation, once you subtract the room the press needs:
n-up = floor(usable sheet width ÷ page width) × floor(usable sheet height ÷ page height)
The “usable” part matters. A press sheet needs grip margin (typically 8–12 mm), and every page needs its own bleed and trim allowance. A sheet that fits 3 A4 pages by pure area will not fit 3 once bleed is added.
| Press sheet | Size (mm) | A4 page | A5 page |
|---|---|---|---|
| SRA3 | 320 × 450 | 2 up | 2 × 2 up |
| SRA2 | 450 × 640 | 4 up | 4 × 2 up |
| B2 | 500 × 707 | 4 up | 8 up |
Note the orientation: A4 is portrait (210 × 297), so two of them side by side need 420 mm of width, which does not fit SRA3’s 320 mm. Two-up A4 on SRA3 works because the pages are rotated to landscape within the sheet.
Every run needs more sheets than the finished quantity, because sheets are lost to make-ready, colour adjustment and spoilage. A common allowance is 3–5% for a straightforward digital run, and more for long offset runs or difficult stock.
Worked example: 500 finished 16-page A5 booklets, one booklet per SRA3 sheet at 4-up, with 5% overs:
Not as a folded booklet. It becomes the next multiple of four with blank pages at the end. If you cannot have blanks, the answer is a different binding method such as spiral or wire-o, where pages are separate and any count works.
A way of printing both sides of a sheet from one plate: the sheet is printed, then turned and fed through again. It halves the plate count and is efficient at high volume, but it constrains the imposition layout, so it has to be decided before the job is planned.
Because the press sheet size, gripper position and folding equipment differ between shops. Imposition is planned against specific equipment, which is why the printer usually generates it rather than the designer.