What does the DPI of an image tell you?
How large the image will print if nobody scales it. It is one number stored in the file, and the print size is the width in pixels divided by it. It says nothing about sharpness or quality on its own: two files with the same pixels and different DPI hold exactly the same picture.
That is why the tool shows three things side by side:
- Current DPI, the number stored in the file.
- Pixel size, the real measure of how much detail there is.
- Print size, pixels divided by DPI, in inches and centimetres.
How big will my image print?
Pixels divided by DPI gives inches; multiply by 2.54 for centimetres. These are common image sizes and how big they print at the two DPI values people ask about most:
| Image size | At 300 dpi | At 150 dpi |
|---|---|---|
| 1080 × 1080 px (square social post) | 3.6 × 3.6 in (9.1 × 9.1 cm) | 7.2 × 7.2 in (18.3 × 18.3 cm) |
| 1920 × 1080 px (Full HD screen) | 6.4 × 3.6 in (16.3 × 9.1 cm) | 12.8 × 7.2 in (32.5 × 18.3 cm) |
| 3024 × 4032 px (12 megapixel phone photo) | 10.08 × 13.44 in (25.6 × 34.1 cm) | 20.16 × 26.88 in (51.2 × 68.3 cm) |
| 6000 × 4000 px (24 megapixel camera) | 20 × 13.33 in (50.8 × 33.9 cm) | 40 × 26.67 in (101.6 × 67.7 cm) |
The stored DPI doesn’t change these sums. If your file says 72 dpi, a print shop still gets the pixels and fits them to the size you ordered.
What does a print shop mean by 300 dpi?
It means 300 pixels for every inch of the finished print. Boise State University’s print shop writes that “all images must be at least 300 dpi at 100% print size” (Boise State Printing). So the check is on pixels per inch at your print size, not on the number stored in the file. Pixels needed at 300 dpi, rounded up:
| Print size | Pixels at 300 dpi | Pixels at 150 dpi |
|---|---|---|
| 4 × 6 in | 1200 × 1800 | 600 × 900 |
| 5 × 7 in | 1500 × 2100 | 750 × 1050 |
| 8 × 10 in | 2400 × 3000 | 1200 × 1500 |
| US Letter, 8.5 × 11 in | 2550 × 3300 | 1275 × 1650 |
| A4, 21 × 29.7 cm | 2481 × 3508 | 1241 × 1754 |
| A3, 29.7 × 42 cm | 3508 × 4961 | 1754 × 2481 |
300 is not a rule for every job. Carleton University’s print shop says sheets of 8.5 × 11 inches or larger need 150 to 300 dpi, and that the DPI needed falls as viewing distance grows (Carleton University Print Shop). Ask your shop what it wants for the size you are ordering.
Why does my file show more than one DPI?
Because several formats can store it in more than one place, and nothing forces them to agree. The tool reads all of them and lists each with its raw value:
| Stored in | Found in | Raw unit |
|---|---|---|
| PNG pHYs chunk | PNG | Pixels per metre (300 dpi is 11,811) |
| JFIF header | JPG | Per inch, per cm, or an aspect ratio with no unit |
| EXIF | JPG, PNG, WebP | Per inch or per cm, as a fraction |
| Photoshop resolution | JPG, TIFF saved by Photoshop | Per inch |
| TIFF tags | TIFF | Per inch or per cm |
| XMP | JPG, PNG, TIFF, WebP | Per inch or per cm, as text |
| BMP header | BMP | Pixels per metre |
A JFIF header with units set to 0 stores only the shape of the pixels, not a DPI; the tool shows it as “aspect ratio only”. A file with no field at all shows “none stored”.
If the values disagree, or the number is not the one your print shop wants, Change image DPI writes one value to every field without touching the pixels.