Change the DPI of an Image, Free and In Your Browser

No, and any tool that suggests otherwise is wrong. DPI is a number in the file header that says how large to print the pixels you already have. Setting a photo to 300 DPI does not add detail; it just tells the printer to put the same pixels into a smaller physical area.

It means 300 pixels make up one printed inch. So a 1200 by 1800 pixel photo at 300 DPI prints at 4 by 6 inches. The same file at 150 DPI prints at 8 by 12 inches, larger but coarser. The tool shows you that print size as you change the value.

No. Only the header is rewritten, so the pixel data comes out byte for byte identical and the file size barely changes. Most DPI tools decode and re-save the image, which quietly recompresses your photo.

Yes. Because the image is never re-encoded, camera settings, dates and orientation all survive. The EXIF resolution tags are updated to match, so Photoshop and Windows report the same DPI as everything else.

JPG and PNG. JPG stores density in its JFIF header and in EXIF, and both are kept in step. PNG stores it in a pHYs chunk. WebP has no standard DPI field at all, so there is nothing to set on a WebP.

It is normal. Screenshots and web images usually declare nothing, and programs then assume their own default, commonly 72 or 96. Setting an explicit value is exactly what a print shop or a submission form is asking for.

Then the pixel count has to change, not just the header. Work out the pixels you need (inches times DPI, so 4 by 6 inches at 300 DPI is 1200 by 1800 pixels), resize to that with the bulk image resizer, then set 300 DPI here.