Where a file keeps its DPI
| Container | Field | Unit stored |
|---|---|---|
| JPEG | JFIF APP0 header (density) | per inch, per cm, or aspect only |
| JPEG | Exif IFD0 XResolution / YResolution | per inch or per cm |
| PNG | pHYs chunk | per metre, or aspect only |
| TIFF | IFD0 XResolution / YResolution | per inch or per cm |
| WebP | EXIF chunk (rare) | per inch or per cm |
| BMP | header pixels per metre | per metre |
| GIF | none | — |
Each field is read on its own and shown with its own unit converted to dots per inch. Nothing is merged: when a JPEG says 72 in one header and 300 in the other, you see both, because that disagreement is exactly what makes two programs print it at two sizes.
A 3000 × 2000 photo labelled 72 DPI
| Resolution | Print size (cm) | Print size (in) |
|---|---|---|
| 300 DPI | 25.4 × 16.93 | 10 × 6.67 |
| 240 DPI | 31.75 × 21.17 | 12.5 × 8.33 |
| 150 DPI | 50.8 × 33.87 | 20 × 13.33 |
| 72 DPI (what the file says) | 105.83 × 70.56 | 41.67 × 27.78 |
The pixels never change down that table. Only the number in the header does, and with it the size a print dialog lays the picture out at.
Limits
- This page reads; it never writes the file. Saving the report gives a .txt, not a new picture.
- A camera's private MakerNote may hold its own resolution figures; only the standard fields above are read.
- A JPEG-compressed or CMYK TIFF opens only where the browser itself decodes it (Safari); elsewhere it is declined by name before anything is read.
Questions
How do I check the DPI of an image?
Open it here. The panel reads every place the container can record a resolution — the JFIF header and the Exif block in a JPEG, the pHYs chunk in a PNG, the first directory of a TIFF, the Exif chunk of a WebP, the header of a BMP — and lists each one with its value and unit, or says it is absent. Nothing is changed and nothing is written unless you choose to save the report.
Why does one app say 72 DPI and another 300 for the same photo?
Because a JPEG can carry the number twice — once in its JFIF header and once in its Exif block — and applications disagree about which to believe. If the two differ, two programs print the same pixels at two sizes. The check shows both values side by side and says whether they agree.
My file says 72 DPI. Is it low quality?
Not by itself. DPI is a label telling a printer how many pixels to put in an inch; the detail is in the pixel count. A 3000 × 2000 photo labelled 72 DPI holds exactly the same picture as the same photo labelled 300 DPI. The print-size table shows what each label would mean on paper.
What if the file records no DPI at all?
Then its print size is up to the application that opens it — many assume 72 or 96. The check says so plainly, and the print-size table still shows what the pixels would give at 300, 240, 150 and 72 DPI.
Can I fix the number here?
This page only reads. To change the label without touching a pixel, use Change DPI or Convert to 300 DPI; if the pixels are genuinely too few for the size you need, Resize for printing tells you by how many.
Neighbouring measurements
Found the wrong number? Change only the DPI — the pixels stay as they are. Need it to print bigger than the pixels allow? The print page says by how much.
- Change DPIRewrite the density tag without touching a pixel.
- 300 dpiThe number a print vendor asks for, written into the file.
- Print size calculatorPixel dimensions, centimetres, inches and density kept visibly linked.
- Resize an image for printingA4, photo and frame sizes: retag, lower DPI, or short by N pixels.
- Why two apps show different print sizesJFIF and Exif can disagree; how to see it and make the file say one thing.
