What DPI Really Controls
Dots per inch tells printing software how large to render the pixels on paper. A 3000px-wide image tagged 300 DPI prints 10 inches wide; retag it 150 DPI and the same pixels print 20 inches wide, softer. On screen, DPI does nothing at all: browsers and phones ignore the tag completely.
That's why "please supply the image at 300 DPI" is a request you can often satisfy without touching the image. If the pixel count is already sufficient for the print size, the file just needs its density field rewritten, and that's the entire job this tool does. JPGs get their JFIF density header updated; PNGs get a correct pHYs chunk. Every pixel byte passes through untouched, so there's no generation loss and the readout will show a size change of a few bytes at most.
Checking you have the pixels
The math is one multiplication: inches of print times target DPI. A 6x4 inch photo at 300 DPI needs 1800x1200 pixels. If your file falls short, retagging won't add detail; you need a bigger source, or accept the lower effective resolution.
| Tag | Typical use |
|---|---|
| 72 DPI | The historical screen default many tools write |
| 150 DPI | Posters and large prints viewed at a distance |
| 300 DPI | The standard ask for documents, journals, and photo prints |
Files in other formats can take a round trip through the converter to PNG or JPG first, then come back here for the tag.