What 300 DPI Really Means (and How to Change It)
DPI is a label, not detail. It tells a printer how many of an image’s pixels to place in each inch. Changing a photo from 72 to 300 DPI changes nothing in the picture; it changes how large it prints. What decides print sharpness is the number of pixels.
The one formula
Print size in inches = pixels ÷ DPI. A 3000 × 2400 image prints at 10 × 8 in at 300 DPI, or 41.7 × 33.3 in at 72 DPI, with exactly the same pixels. For centimetres, multiply the inches by 2.54.
Pixels you need at 300 DPI
| Print size | Pixels at 300 DPI | Pixels at 150 DPI (posters viewed from afar) |
|---|---|---|
| 2 × 2 in passport photo | 600 × 600 | 300 × 300 |
| 4 × 6 in (10 × 15 cm) | 1200 × 1800 | 600 × 900 |
| 5 × 7 in | 1500 × 2100 | 750 × 1050 |
| 8 × 10 in | 2400 × 3000 | 1200 × 1500 |
| A4 (210 × 297 mm) | 2480 × 3508 | 1240 × 1754 |
| US Letter (8.5 × 11 in) | 2550 × 3300 | 1275 × 1650 |
When the DPI number matters
- Forms and print shops that check for “300 DPI” read the number stored in the file. Setting it with Change Image DPI passes that check without touching the image.
- Word, InDesign and similar programs use it to decide the size an image appears when placed: a 72 DPI photo appears four times larger than at 300 DPI.
- Screens ignore it. A website or phone shows pixels, whatever the DPI says.
“Increase DPI to improve quality” is a myth. To print larger and sharp you need more pixels from the camera or scanner. Upscaling software guesses new pixels; it cannot recover detail that was never captured.
Where DPI is stored
In a JPG it sits in the JFIF header and often again in the EXIF data (some programs read one, some the other); in a PNG it is the pHYs chunk. WebP and HEIC have no standard DPI field. Our tool updates both JPG fields and the PNG chunk, and shows what a file currently stores.