DPI converter

DPI is metadata, not resolution, and that is the one thing worth taking away. An image is a grid of pixels; the DPI is a note attached to it saying how large to print that grid. A 3000-pixel-wide photo is 10 inches at 300 DPI and 30 inches at 100 DPI — the same pixels either way. Changing the number changes the intended print size and nothing about the image.

This is why "make this 300 DPI" is an incomplete request, and why so much confusion surrounds it. If a printer asks for 300 DPI they mean 300 pixels for every inch of the final printed size — so a 6 by 4 inch print needs 1800 by 1200 pixels. Setting the DPI field on a 600-pixel image to 300 does not create detail; it just says "print this at 2 inches wide". The tool shows the print size at both the current and the new DPI for exactly that reason.

The write is lossless because it edits a header rather than re-encoding. JPEG stores density in the JFIF APP0 segment; PNG stores it in a pHYs chunk, in pixels per metre rather than per inch. Both are a handful of bytes near the start of the file. Doing the same by opening and re-saving in an editor would recompress the entire image to change one number, which for a JPEG loses quality every time.

Where the field is ignored entirely: screens. A browser, a phone and a monitor all disregard DPI completely and show the pixels. It only affects print and page-layout software.