How much bleed do I need for printing?
Short answer
Bleed is artwork extended beyond the trim line so that cutting variance does not leave an unprinted edge. Typical requirements are 3 mm for digital and commercial print, and 10 mm or more for large format where finishing needs extra material.
Why it exists
Cutting is mechanically accurate to a tolerance, not absolutely. If artwork stops exactly at the trim line, a variance of a fraction of a millimetre shows the substrate. Extending the artwork past the trim means the cut always falls inside printed area. The same logic applies to contour cutting, where the tolerance is often larger.
Trim size 100 × 150 mm Bleed 3 mm all round Document 106 × 156 mm Safe margin 4 mm inside trim
When bleed can be added afterwards
When the artwork runs to the trim edge, the edge can be mirrored outward, or artwork the file already holds past the trim can be shown, without moving anything inside the trim. That can be automated, and checked by rendering. It is the most common single repair in prepress.
When it needs a person
Where the design ends before the trim — a white border, a photograph that stops short — there is nothing to extend, and nothing needs to be: the bleed stays blank paper. Where text, a logo or a border sits right at the edge, a mirror repeats it back to front in the bleed, and a cut that drifts outward shows it. Someone has to decide: accept it, ask for a file built with bleed, or agree a visible margin instead.
How Prepta handles it
Prepta adds the bleed your profile requires to images and PDFs. In a PDF it uses artwork the file already holds past the trim, and otherwise mirrors the edge outward; the artwork inside the trim stays where it is, and a render check confirms it. In a PDF, where live text or a line sits in the mirrored strip, the result goes to a person before production. Every change is listed with the job, and mirrored bleed is listed as reconstructed.
Sources and further reading
Standards and vendor documentation behind the statements above.