In the design of print jobs it is necessary to take in account: (a) the thickness of the substrate (usually paper), and (b) the tolerances of the printing machines. Even a simple business card must pass through two, and possibly three machines; and a folded brochure three or even more machines. Typically these machines are accurate in cutting, printing, folding, perforating, etc. only to + or - 1 mm.
Let’s take a look at what a bleed actually is, why they are required, and how to set them up.
This is the area of artwork that is extended beyond the actual dimensions of the document. It is used to avoid strips of white paper showing on the edges of your print should the batch be misaligned when cut to size.
Any objects in your artwork that touch the edges of the document require bleed, for instance a background colour or image should spread to cover the entire bleed area as should any objects that creep in from the side of the page.
A bleed of 0.125" (about 3 mm) is commonly required. This 3 mm accounts for each of the four sides of the page, therefore you should add 6 mm to the width and height of the document, for example:
8.5"x11" Paper = 216 mm x 279 mm
216 mm + 6 mm = 222 mm
279 mm + 6 mm = 285 mm
Total Document Size = 222 mm x 285 mm
In addition to bleed, you should also add a margin to the edges of the document to avoid having your objects look as if they are about to fall off the page or even worse actually get cropped off when the document is trimmed! The amount of margin should be at least 3 mm and preferably 5 or 6 mm. Margins which are less than this will be treated as bleeds and incur all the associated expenses—typically an extra 30% in cost.