What DPI actually means
DPI — dots per inch — is a small instruction stored inside an image file that tells a printer how densely to place the pixels on paper. It changes no pixels at all; it only sets the intended print size. The maths is one line: print width = pixel width ÷ DPI. A 3000-pixel-wide photo prints at 41.7 inches at 72 DPI, and at exactly 10 inches at 300 DPI. Same image, same detail — the DPI value just decides how big it lands on the page.
Changing the DPI does not improve quality
This is the part most converter sites skip, so let's be direct: rewriting 72 DPI to 300 DPI adds no detail and no sharpness. Your pixel count is exactly what it was. If a photo has too few pixels for the print size you want, setting 300 DPI makes the printed result smaller at the same softness rather than sharper at the same size. The only real fixes are starting from a higher-resolution original, or printing smaller. (Actually changing pixel dimensions is a different job — that's our Resize Image tool, and upscaling has the same can't-invent-detail limit.)
DPI maths: how big will it print?
For a 1200 × 1800 pixel photo (a typical "4×6 at 300 DPI" file):
| DPI setting | Print width | Print height |
|---|---|---|
| 72 DPI | 16.7 in (42.3 cm) | 25 in (63.5 cm) |
| 150 DPI | 8 in (20.3 cm) | 12 in (30.5 cm) |
| 300 DPI | 4 in (10.2 cm) | 6 in (15.2 cm) |
Run it backwards to sanity-check a print order: a shop asking for a 10-inch-wide print at 300 DPI needs roughly 3000 pixels across. If your image is 1500 pixels wide, it will be printed at half that sharpness no matter what number is written in the DPI field — better to order a 5-inch print, which lands at a true 300 DPI.
When print shops and upload portals ask for "300 DPI"
Print shops, photo labs, and print-on-demand portals ask for 300 DPI files because that's roughly the density at which prints look sharp at reading distance. Many check the file's DPI metadata (or the effective DPI at your chosen print size) on upload — which is why a perfectly good photo can bounce back with a "low resolution" warning. If your pixels genuinely cover the size, setting the DPI flag to 300 with this tool fixes the file honestly. If they don't, the warning is telling you something real: print smaller or use a bigger original. ID and visa photos have their own pixel rules — see the Passport Photo Compressor.
DPI vs PPI — is there a difference?
Near enough, no. PPI (pixels per inch) describes displays and the pixels an image has; DPI describes printer dots. Image files store one density number, and different apps label it DPI or PPI — this tool sets that shared value. Screens ignore it completely, so changing DPI does nothing to how an image looks on a website or phone; it only matters the moment something is printed.
Which formats keep a DPI setting?
JPEG stores it in its JFIF header and PNG in a dedicated chunk — this tool rewrites those exact bytes, so the picture data is byte-for-byte untouched and the file size barely moves. WebP and other formats don't carry the same standard value, so this tool hands those back as a PNG copy with the DPI set. Order matters if you're also shrinking the file: set DPI last, after any resizing or compressing with tools like Compress Image, because re-encoding can rewrite metadata.