Changer le DPI d’une image

DPI only tells software how many pixels to place on each inch of paper. Changing it adds no detail. A 1800 × 1200 photo at 300 DPI prints at 6 × 4 inches (15.24 × 10.16 cm); set to 150 DPI, the same pixels print at 12 × 8 inches, each one twice as large. The table on this page shows the print size before and after for the selected file.

So when a form asks for 300 DPI, check two things. First, the label: many phone photos store 72 or nothing, and some upload checks reject them on that alone. This page fixes that. Second, the pixel count: for a sharp 8 × 10 inch print at 300 DPI you need 2400 × 3000 pixels. If the photo is smaller, changing the label makes the print smaller, not sharper.

In a JPG the resolution lives in the JFIF header. The page sets its unit to inches and both densities to your value; if the file has no JFIF header, a standard 18-byte one is added after the start marker. Cameras and editors may also store resolution in EXIF (XResolution, YResolution, ResolutionUnit) and Photoshop in its own resolution block, which Photoshop reads first. When those fields exist, they get the same value, so no program is left reading an old number from a second field. Missing EXIF fields are not added, and XMP metadata is left as it is.

A PNG keeps resolution in a pHYs chunk measured in pixels per metre: 300 DPI is stored as 11,811. The chunk is rewritten in place, or a new 21-byte chunk is inserted before the image data, with a fresh checksum.

The 1200 × 800 test photo used on this site (a 224,092-byte JPG with a JFIF header) is still 224,092 bytes at 300 DPI, and its decoded pixels match the original exactly. The 1,601,750-byte PNG test file had no pHYs chunk, so it grows by 21 bytes to 1,601,771, again with identical pixels. The site’s own image size checker reads the new value back as 300 dpi.

Other formats are refused with a message: WebP and AVIF store resolution differently or not at all, so convert them to JPG or PNG first.