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Change image DPI
DPI is a number written in the file header. It does not describe the image; it describes how large a printer should make it. Changing it is either free or expensive, depending on which of the two things below you actually want.
Result
Retagging versus resampling
Retagging rewrites the density value and nothing else. The pixel data is copied across untouched, so there is zero quality loss and the file size barely moves. What changes is the physical size the image claims: a 1200 pixel wide photo tagged at 300 DPI prints at four inches, and the same file tagged at 150 DPI prints at eight. This is what you want when a print shop or a journal says "we need 300 DPI" and your image already has enough pixels.
Resampling holds the physical size constant and changes the pixel count to match the new density. Going from 72 to 300 DPI this way means multiplying the pixel count by more than four, and those pixels have to be invented. The file gets much bigger and the image gets softer. Occasionally a submission system demands it anyway.
The honest version of "increase DPI to 300"
When a publisher asks for 300 DPI, what they mean is that they want enough pixels for the size they intend to print. Retagging a small image to 300 DPI satisfies the letter of the request and produces a small print. Resampling satisfies the number but not the intent, because the detail is not there. The real answer, when there is one, is to go back to the original file at full resolution.
Common questions
How do I check the DPI of an image?
Drop it in above. The panel on the right reads the JFIF header for a JPEG or the pHYs chunk for a PNG and reports what it finds. Many files have no density tag at all, in which case software falls back to 72 or 96.
Does changing DPI affect image quality?
Retagging does not, in any way. Resampling does, because it rebuilds the pixel grid. The tool tells you which one happened.
Does DPI matter for images on a website?
No. Browsers ignore the density tag entirely and lay out images by pixel count. It only matters when something is being printed or placed in a page layout application.
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