Bleed is a word used in print design in order to explain when printing expands over the edges of the document. Every printer has some kind of bleed set on the sides of the document so that the printer has a small amount of space to move around the paper. It is possible for you to change the bleed settings on your printer should you need to do so for a certain project. Otherwise, the default, and most widely used bleed is at 3mm.
Each country has its own way of measuring the bleeds though, so this depends on the rules that your country uses. The United States and the United Kingdom both use inches to measure bleeds, so the standard bleed in these countries is 1/8 inch from the area that is to be cut. While in Europe, millimeters are used and 3mm to 5mm is the default measurement for bleeds.
The length of bleed that you want for your finished project will depend largely on what you are printing. For example, let’s say you are printing a 4×6 postcard. The graphics and image should be extended by 1/8 of an inch on all sides. The image itself should be created with a measurement of 4.25 inches by 6.25 inches. Then, once the postcard has been printed out, the 1/8 inch bleed will be trimmed, leaving you with the finished product.
The reason why leaving some room for a bleed is so important is due to the fact that it allows you some room for error. There are many possible errors that could occur during the printing process. For example, the wrong size paper could be used, or the cropping machine could even be set up wrong. Images will not be aligned as nicely if you do not allow for a bleed either.