Change Image DPI

DPI is the most misunderstood number in imaging, and the confusion is profitable for tools that pretend otherwise. An image is a grid of pixels - say 3000x2000 - and that grid is all the picture data there is. DPI (dots per inch) is a note in the file header suggesting how densely to lay those pixels onto paper. At 300 DPI, 3000 pixels span 10 inches; retag the same file to 150 DPI and the identical pixels span 20 inches. Nothing about the image itself changed - only the printing suggestion.

In a JPEG, that suggestion lives in the JFIF header as a pair of density fields - two bytes each for the X and Y values, plus a units flag. PNG stores the equivalent in a small chunk called pHYs, expressed as pixels per meter. Changing DPI means rewriting those few bytes and nothing else, which is why this tool finishes instantly, why the file size barely moves, and why "quality loss" is impossible here by construction.

So why do portals and printers demand 300 DPI? It is shorthand for a real requirement they phrase badly: enough pixels for the intended print size. A 4x6 inch print at 300 DPI needs 1200x1800 pixels. If you have those pixels, tagging the file 300 DPI makes every automated checker happy. If you do not - if a form wants a 2-inch 300 DPI photo and your image is 300 pixels wide - retagging cannot conjure the missing 300 pixels; resize (upward, with interpolation) first, then set the DPI.

A final source of confusion: screens ignore DPI entirely. A 96 DPI and a 300 DPI copy of the same image render identically in every browser and viewer, because displays map image pixels to screen pixels without consulting the header. DPI matters at exactly two moments - printing, and passing a metadata check.