DPI is a label; pixels are the picture
DPI (dots or pixels per inch) is a number stored in the file that tells printing software how large to print it. A 3000 × 2000 picture labelled 300 DPI prints at 10 × 6.67 inches; the same pixels labelled 72 DPI would print at 41.7 × 27.8 inches. Screens ignore DPI completely.
With “Change DPI only” ticked, a JPEG or PNG is patched in place: the JFIF header or pHYs chunk is rewritten and the compressed picture data is copied byte for byte. Nothing is re-encoded and nothing is lost.
Scaling for a print size
Untick it to resample instead. Enter the print size in centimetres, millimetres or inches and the DPI the printer wants; the tool computes the pixels (15 × 10 cm at 300 DPI is 1772 × 1181), scales to them and writes the DPI into the saved file.
Questions
How do I make an image 300 DPI?
Open it, keep “Change DPI only” ticked, choose 300 and download. For JPEG and PNG the result is identical pixel for pixel with the DPI field changed. Other formats are saved as PNG with the new DPI.
Does changing DPI improve print quality?
Not by itself — it only changes the printed size. If the print is too small at 300 DPI, untick “Change DPI only” and scale up, ideally with the AI engine.
Is DPI written into WebP files?
No. WebP has no standard DPI field that printing software reads, so DPI is written for JPEG and PNG only, and the panel says so.
What DPI do photo labs and forms want?
Photo labs commonly ask for 300 DPI. Application forms often specify pixel dimensions and file size rather than DPI; confirm the exact requirement on the official portal.
Is my picture uploaded anywhere?
No. Decoding, scaling and encoding all run in this browser tab, and the scaling engine runs in a Web Worker with no network code in it. The only download the site can make for you is an AI model file, and only after you press the button that names its size — that request contains no picture.