Where a JPEG keeps its DPI
A JPEG can hold the DPI in more than one place. The JFIF block near the start of the file holds a density value. The Exif block, written by cameras and phones, can hold X and Y resolution. Photoshop adds a third copy in its own block.
The tool updates every one of these that exists. If the file has no JFIF block, it adds one right after the start of the file. This keeps different programs in agreement, so Preview, Windows and print software all show the same number.
Photos from a phone or camera
Camera files often say 72 DPI or 300 DPI in Exif and have no meaning for print. A 4000 by 3000 pixel photo prints at 13.3 by 10 inches at 300 DPI, whatever the label says. The row shows that size so you can check it before you download.
Phone photos stored sideways with an orientation tag are read correctly, so the width and height in the row match what you see on screen.
A rare exception
Some JPEG files saved as RGB by Adobe tools would change color if a JFIF block were added. If such a file has no other place for a DPI value, the tool tells you and does not change it, instead of shifting the colors.
Questions
- Will my JPG lose quality?
- No. The compressed picture data is copied without any change, so the image is identical.
- Is JPG the same as JPEG?
- Yes. They are two spellings of the same format. Both .jpg and .jpeg files work here.
- Can I read the DPI of my photo here?
- Yes. Add the file and the row shows its DPI now, its pixel size and its print size.