Bleed

The problem it solves

Printing presses cut stacks of paper, and paper shifts fractionally during cutting. If your design ends exactly at the trim line, a shift of half a millimetre leaves a hairline of unprinted white along one edge. Across a print run some copies are perfect and some are not, which is worse than a consistent margin.

Bleed removes the possibility. The artwork is extended past where the page will be cut, so the blade passes through printed area no matter where it lands, and the excess is discarded.

How much

The North American convention is 0.125 inches on every side; the European convention is 3 millimetres. At 300 DPI that makes a US Letter page 2625 x 3375 pixels for a document that finishes at 2550 x 3300, and an A4 page 2516 x 3544 for one that finishes at 2480 x 3508.

The bleed is not part of the design. Nothing you want to see should live in it, because all of it is going in the bin.

The safe margin, which is the other half

Bleed protects against the cut landing outside the page. The safe margin protects against it landing inside. Keep text, logos and anything that must not be clipped at least 5 millimetres — or half an inch, in US work — inside the trim line.

Between the two, roughly the outer 8 millimetres of a design is either discarded or at risk. Binding takes more: a perfect-bound spine can consume 10 millimetres or more of the inner edge before anything is readable.

When you do not need it

A design with a white background and generous margins needs no bleed at all, because there is no colour that has to reach the edge. That is an honest reason to design one, particularly for documents that will be printed in an office rather than at a print shop — office printers cannot print to the edge anyway, and their drivers silently scale a full-bleed page down to fit, shrinking every element.