Changing DPI does not add pixels
DPI is commonly used to describe an image’s print-density setting; for image pixels, PPI is the more precise term. The stored number tells a print application how densely to place those pixels on paper. It does not create more detail or make a blurry photo sharp.
A 1200 × 1800 px image at 300 DPI has a nominal print size of 4 × 6 inches. Change its tag to 600 DPI and the nominal size becomes 2 × 3 inches. The image still has the same 1200 × 1800 pixel grid.
If you need both a particular physical size and density, calculate the necessary pixels first or use Resize Image in print mode. Enlarging a small image can produce more pixels, but interpolation cannot recover detail that was absent from the source.
Print-size calculations
| Calculation | Formula |
|---|---|
| Inches from pixels | Pixels ÷ DPI |
| Centimetres from pixels | Pixels ÷ DPI × 2.54 |
| Pixels for a print width | Inches × DPI, rounded to a whole pixel |
| Pixels for a width in cm | Centimetres ÷ 2.54 × DPI, rounded |
At 300 DPI, a 3.5 × 4.5 cm image needs approximately 413 × 531 pixels. The 3.5 × 4.5 cm resizer can prepare that pixel grid. Simply adding a 300 DPI tag to a different-sized photo will not produce that exact physical size.
How JPG and PNG store resolution
For JPG, the tool writes horizontal and vertical JFIF density values in dots per inch. Private EXIF and XMP details are removed first so an old EXIF resolution value does not conflict with the new density. Essential orientation and colour information are preserved.
PNG stores density as a whole number of pixels per metre, so a requested value such as 300 DPI is represented by the closest available pixels-per-metre value. A separate application may display a tiny decimal rounding difference, such as 299.999 DPI.
Applications can ignore density tags, fit an image to a page or apply a printer setting of their own. Check the print dialog and disable unwanted fit-to-page scaling when physical size matters. For online portals, follow the stated pixel and KB limits; a DPI tag alone does not satisfy them. Use the Image Size Checker after downloading.