Set the DPI label a form or a printer reads — 72, 96, 150, 200 or 300 — without touching a pixel of a JPEG. Or resample to a print size in cm or inches at that DPI, when the pixels have to change too.
Two ways to change DPI
Write DPI only. With the DPI label set and the lossless switch on, a JPEG keeps its compressed picture exactly; the JFIF header's density and the EXIF X/Y resolution are rewritten to agree, and that is all. This is what “change DPI” means to a form that checks the number.
Resample to a print size. Choose Print in the Size panel, type the paper size and the DPI, and the photo is resized to exactly that many pixels and labelled. Use this when the form asks for a physical size; it re-encodes, because the pixels are new.
Step by step
- Drop your JPG (or several). Other formats work too; they are converted once.
- In Size, 300 dpi is already chosen with Write DPI only on. Pick another value if the form asks for it.
- For a physical size, choose Print, type the size in cm or inches and the DPI.
- Press Download. The row reads the DPI back out of the written file.
Why two labels
A JPEG can state its density twice: in the JFIF header near the start, and in the EXIF block a camera writes. Different programs read different ones, and a file where they disagree shows different DPI in different places. Both are written here, and both are read back.