A worked example
A 3000 × 2000 JPG from a phone says 72 DPI, so a print shop's software proposes 105.8 × 70.6 cm. Set 300 and save: the file still has 3000 × 2000 pixels, its JFIF header now says 300, and the same software proposes 25.4 × 16.9 cm. The saved file is the same size, give or take 18 bytes if the JPG had no JFIF header and one had to be added.
| Pixels | At 72 DPI | At 150 DPI | At 300 DPI |
|---|---|---|---|
| 1200 × 1800 | 42.3 × 63.5 cm | 20.3 × 30.5 cm | 10.2 × 15.2 cm |
| 2480 × 3508 | 87.5 × 123.8 cm | 42.0 × 59.4 cm | 21.0 × 29.7 cm (A4) |
| 3000 × 2000 | 105.8 × 70.6 cm | 50.8 × 33.9 cm | 25.4 × 16.9 cm |
When another page here fits better
If the print needs more pixels than the file has, change the pixels on resize image for print, which shows the effective DPI of your own pixels at each paper size. Pixels and print size explains the arithmetic.
What stays on your device
The file is read into this tab, a few header bytes are rewritten, and the result is handed to your downloads folder. Nothing — not the file, not its name, not the DPI — is sent anywhere.
Only JPG and PNG carry the tag. A file whose EXIF says one DPI and whose JFIF header says another is set to the new value in both places that exist; this page does not add an EXIF block that was not there.
Questions people ask here
Does 300 DPI make a picture sharper?
No. DPI only tells a printer how far apart to put the pixels. A 3000-pixel-wide photo prints 105.8 cm wide at 72 DPI and 25.4 cm at 300 — the same pixels, packed closer. Sharper prints need more pixels, which resize image in cm can give you, honestly labelled as an enlargement.
Is the quality reduced?
No. The file is never decoded and saved again. The page copies the compressed image data across unchanged, and the Checked list shows the SHA-256 of that data before and after: the two are the same.
Does WebP have DPI?
Not as a resolution tag. WebP files carry no DPI field, so there is nothing to write; save the picture as JPG or PNG on the resizer and set DPI in the file there.
My form asks for 200 DPI. Is this enough?
If the pixel size and file size already match the notice, yes — this page changes only the tag. If not, the exam-form page sets the pixels and kilobytes and offers to write the DPI in one go.