What 300 DPI actually means once it is on fabric
DPI only means something next to a size. Here is how to tell whether your file is big enough for a 12 by 16 inch chest print, and what happens below 150.
DPI on its own tells you nothing. It is a ratio, and the other half of the ratio is the printed size. A file at 300 DPI is only 300 DPI at one particular width, and if you scale that file up to cover a chest, the number falls with it.
The number that matters is pixels
A standard adult chest print here is 12 inches wide. At 300 dots per inch that needs a file 3600 pixels across. That is the whole calculation: inches times DPI equals pixels. A 1200 pixel image stretched across 12 inches is printing at 100 DPI, whatever the file properties claim.
| Print width | At 300 DPI | At 150 DPI |
|---|---|---|
| 12 in chest | 3600 px | 1800 px |
| 4 in pocket | 1200 px | 600 px |
| 50 in blanket | 15000 px | 7500 px |
Why 150 is the floor and not the target
Ink spreads slightly into the weave, which hides a little softness that would be obvious on paper. That is why 150 DPI on a shirt still looks acceptable at arm's length while 150 DPI on a poster looks wrong. Below 150 the spread stops helping and edges start to look chewed, particularly on thin lettering.
The print sheet that goes to the factory grades every file. Below 150 DPI it prints a red band telling the printer not to run the job, because a soft print gets reprinted at our cost rather than yours.
Enlarging does not add detail
Resampling a small image up to 3600 pixels produces a 3600 pixel file that still contains the detail of a small one. The number in the file properties changes and nothing else does. If the original is small, the honest options are to use it smaller, or to redraw it.
Where this bites
- Photographs saved from a chat app, which are usually resized down on the way
- Logos exported at screen size, typically 500 pixels or less
- Anything screenshotted rather than exported
- Art scaled up in the editor after it was placed, which enlarges without adding anything
Vector art avoids the problem entirely, because it has no pixels until the moment it is printed. If you have an SVG version of a logo, use that one.