Work out document size, trim and safe area for any finished size, with a to-scale diagram whose SVG is measured in real millimetres.
Three millimetres is not an eighth of an inch
An eighth of an inch is 3.175 mm, which is 5.83 per cent more than 3 mm. They are used interchangeably in conversation and they are not interchangeable in a document. An A4 flyer set up with 3 mm bleed is 216 × 303 mm; the same flyer at an eighth of an inch is 216.35 × 303.35.
That difference will usually survive a print run, because the trimming tolerance is larger than the discrepancy. But it will fail a preflight check that tests for an exact document size, and several printers do exactly that. Use whichever figure your printer publishes, literally.
Per edge, or across the whole dimension?
Most printers state bleed per edge, which is what this calculator assumes and what almost everyone means. Two significant ones do not. 4over's own instruction reads "enter .125″ as the bleed for a total of .25″", and GotPrint states it the same way.
Read their figure as per-edge and you will set up a document with twice the bleed they asked for. It is worth thirty seconds on your printer's spec page to check which convention they use, because nothing in the file itself will reveal the error.
Why bleed exists, with the number nobody publishes
The machinery is not the problem. A modern guillotine positions its knife to 0.01 mm and a press holds colour-to-colour registration at about 0.10 mm. Yet the accepted commercial trimming tolerance is around ±0.76 mm, and trade printers routinely publish one to two millimetres.
The error is in the paper, not the machine. A guillotine cuts a lift of several hundred sheets at once and the knife drags them as it descends, so the sheets in the middle of the lift move relative to the ones at the top. ISO 16762 makes the point formally: tolerances accumulate across successive processes rather than resetting.
This is also why anything registered to the printed sheet afterwards is worse again. Die-cutting and foiling land within about a millimetre, spot UV within half of one. If your design depends on a border of exactly even width, it will not survive any of those.
The safe margin is not standardised at all
Bleed at least clusters around two values. The safe margin — how far inside the trim your text must stay — runs from 1.59 mm at Moo to 12.7 mm at Lulu, a factor of eight, for what is nominally the same instruction.
A designer working to a habitual 3 mm is comfortably out of spec at Mixam US, Blurb, KDP and Lulu. Five millimetres is a defensible default for flat work and it is not a rule.
Asked often
How much bleed do I need?
Whatever your printer publishes, taken literally. Across twenty printers it runs from 1 mm to 3.175 mm — a factor of over three for the same product. Three millimetres suits most of Europe and an eighth of an inch most of North America.
Is 3 mm the same as 0.125 in?
No. An eighth of an inch is 3.175 mm, 5.83% larger. It rarely matters on press and it can fail an automated preflight check.
What if my artwork has a white border?
Then you technically need no bleed — but you do need the safe margin, and a border of even width is the hardest thing to hold. Trimming moves by up to about 0.76 mm, so a 3 mm border can come back 2.2 mm on one side and 3.8 on the other.