DPI, bleed and safe areas: the three numbers behind every good print
These three terms account for most of the back-and-forth between designers and printers. None of them is complicated. All of them are easier to understand with the physical reason behind them.
DPI, and why 300
DPI is dots per inch: how many dots of ink the printer lays down across one inch. Printing is generally done at 300, because that is roughly where the human eye stops seeing individual dots at normal viewing distance.
What matters is DPI at final size, not the number stored in the file. An image that is 3,000 pixels wide printed at 10 inches is 300 DPI. The same image printed at 20 inches is 150 DPI and will look soft. Nothing about the file changed; the print got bigger.
So the useful calculation is: final width in inches, times 300, equals the pixels you need.
- 4 inch print: 1,200 pixels
- 10 inch print: 3,000 pixels
- 16 inch print: 4,800 pixels
Large format is the exception. A banner viewed from ten feet away is fine at 100 DPI, because you are not standing close enough to see the dots. The closer something will be viewed, the more DPI it needs.
Bleed, and why the design runs off the edge
Bleed applies when colour goes right to the edge of a printed item - a business card, a sticker, a poster. Printing happens on a larger sheet which is then cut down, and cutting has a tolerance of a millimetre or two.
If your background stops exactly at the edge of the card, a cut that lands a millimetre outside leaves a thin white line along that edge. To prevent it, you extend the background past the trim line, usually by 3mm or an eighth of an inch. That extra is the bleed. It gets cut off, which is the point: the cut lands somewhere within your extended colour and there is no white line wherever it falls.
Bleed is for background only. Nothing you want to keep goes in it.
Safe area, the same idea in reverse
If the cut can land 2mm outside, it can also land 2mm inside. So there is a margin just inside the trim line where you do not put anything important. That is the safe area, typically 3 to 5mm in.
Text is what suffers. A line of text set 1mm from the edge looks balanced on screen and looks like a mistake on a quarter of the run, because on a quarter of the run it is 1mm closer to the edge or clipped.
On garments the same principle applies for a different reason: the garment shifts slightly on the press, so anything close to the edge of the print area may drift.
Putting it together
For a business card with a coloured background:
- Card size 3.5 by 2 inches
- Add bleed: build the artwork at 3.75 by 2.25 inches, with the background filling all of it
- Safe area: keep all text within 3.25 by 1.75 inches, centred
- Resolution: 3.75 inches at 300 DPI means 1,125 pixels wide
For a 10 inch garment print:
- No bleed, because nothing runs off the edge of a shirt
- Keep text a good half inch inside the print area
- 3,000 pixels wide at 300 DPI
Why vector sidesteps most of this
Vector artwork stores shapes as maths rather than pixels, so it has no resolution and prints sharp at any size. Logos and text should be vector wherever possible. Photographs cannot be, which is why photographic prints need the DPI calculation and logos generally do not.