What is DPI, and why changing it does nothing to your image
DPI is dots per inch: how many dots a printer lays down across one inch of paper. It is a fact about printing, not about your file. A digital image is a grid of pixels and has no inches in it at all.
The number stored in a file marked “300 DPI” is a note saying how big it would like to be printed. Here is the same 600 by 400 image saved three times on 2026-09-13, changing only that note:
| Saved as | Pixels | File size | Pixels changed |
|---|---|---|---|
| 72 DPI | 600 x 400 | 12,254 bytes | the original |
| 300 DPI | 600 x 400 | 12,254 bytes | 0 of 240,000 |
| 1200 DPI | 600 x 400 | 12,254 bytes | 0 of 240,000 |
Identical size, identical pixels. Setting an image to 1200 DPI adds nothing and removes nothing. If somebody sent back a file because it was “only 72 DPI”, changing the number would have satisfied them and improved the image not at all.
What they actually meant
Almost nobody asking for 300 DPI wants the metadata. They want enough pixels that the image still looks sharp at the size it will be printed, and DPI on its own cannot tell you that. Pixels divided by inches can.
| Printed size | Pixels at 300 DPI | At 150 DPI | At 72 DPI |
|---|---|---|---|
| Passport photo, 2 x 2 in | 600 x 600 | 300 x 300 | 144 x 144 |
| 6 x 4 print | 1800 x 1200 | 900 x 600 | 432 x 288 |
| A4 page | 2481 x 3507 | 1240 x 1754 | 595 x 842 |
| A3 poster | 3507 x 4962 | 1754 x 2481 | 842 x 1191 |
Read it the other way and it answers the more common question. A 12 megapixel phone photo is 4032 by 3024, which at 300 DPI prints 13.4 by 10.1 inches, comfortably past A4. A 1920 by 1080 screenshot prints 6.4 by 3.6 inches and no further, which is why a screenshot dropped into a document looks soft on paper and fine on screen.
So how do you give somebody a 300 DPI image
Ask what size it is being printed, multiply each side by 300, and make sure the image has at least that many pixels. If it does, set the DPI tag to 300 and you are done, because the pixels were always the thing that mattered.
If it does not have enough pixels, no setting fixes it. Enlarging adds pixels by guessing at what would have been between the ones you have, which is why an upscaled image looks soft: the detail was never recorded. Rescan it, reshoot it, or find the original.
DPI, PPI, and why nobody corrects it any more
Strictly, PPI is pixels per inch on a screen and DPI is ink dots per inch on paper, and a printer uses several dots to make one pixel. The distinction matters if you work in prepress. Everywhere else the two words are used for the same idea, and the person asking you for a “300 DPI photo” will not be helped by the correction.
One place the number genuinely does nothing: the web. A browser lays out images in pixels and ignores the DPI tag entirely. An image is not “72 DPI for web”; it is whatever pixel size you gave it.
Check or change the pixels
These run in your browser, with nothing uploaded. The resizer shows the pixel size of whatever you drop on it, which is the number the tables above are about.
filetity is built by Adarsh Mishra. The three test files above were written with Pillow on 2026-09-13 and compared pixel by pixel; the print tables are arithmetic you can check with a calculator. Corrections welcome: support@filetity.com.