Change image DPI

DPI is a label, not a property of the pixels

This confuses almost everyone, and understanding it saves a lot of wasted effort. An image is a grid of pixels. Nothing about those pixels is "300 DPI" or "72 DPI" — the resolution is a number stored in the file's header that says how large the picture should be printed. A 1200-pixel-wide image tagged 300 DPI prints four inches wide; the same file tagged 150 DPI prints eight inches wide, at half the quality. The pixels are identical in both cases.

So when a form or a print shop asks for 300 DPI, there are two quite different things they might mean, and it is worth knowing which.

"Change the DPI to 300"

Usually this is a checkbox on somebody's submission system reading the header field. Nothing needs to happen to the picture at all — the tag needs to say 300. That is the default here, and it rewrites only the header: the file comes back byte-for-byte identical apart from a few bytes of metadata, with every pixel untouched.

This matters because the usual advice — open it in an editor and re-save at 300 DPI — re-encodes the JPEG and throws away real image quality to change a number that was never about quality. Doing that repeatedly visibly degrades a photograph.

"It needs to be 300 DPI at 4 inches"

That is a genuine resolution requirement and needs actual pixels: four inches at 300 DPI is 1200 pixels wide, full stop. Choose the resize option, give the printed width you need, and the image is resampled to match and tagged to suit. If your original is smaller than the target, it will be enlarged and will look soft — no tool can add detail that was never captured, and one that claims to is upscaling by guesswork.

How the two formats store it

JPEG keeps it in the JFIF header as a density plus a unit — inches or centimetres — which is a straight rewrite of six bytes. PNG keeps it in a pHYs chunk in pixels per metre, and every PNG chunk carries its own CRC-32 checksum, so the chunk has to be rebuilt and checksummed rather than poked. Both are handled properly here; the resulting PNG passes a standard checksum validation.

WebP and AVIF have no equivalent field at all, so if you need a resolution tag you need JPEG or PNG. The tool says so rather than silently returning an unchanged file.

Common values

300 DPI is the print standard and what almost every submission form asks for. 600 is used for line art and archival scanning. 72 and 96 are screen conventions with no real meaning on modern displays, and are mostly encountered because something defaulted to them.

Nothing is uploaded

The header rewrite happens in your browser. For the tag-only mode you can verify the pixels are unchanged by comparing file sizes: the output differs from the input by a handful of bytes.