What DPI should I use?

300 for photos and documents. 150 is fine for posters seen from a distance. 72 only makes sense on screens.

Does DPI change the image?

DPI is only a label for print size. What matters is the pixel count, which this tool sets from your size and DPI.

WebP has no standard place to store DPI, so it would print at the wrong size.

On, the image fits inside your size and nothing is cut. Off, it's cropped from the centre to fill the size exactly.

No. Resizing happens on your device. Nothing is sent to a server.

Resize to cm or mm

Size an image for printing, with the right pixels and the DPI stored in the file.

Runs on your device. Nothing is uploaded.