Inspect an image file

It reads the first bytes of the file. Every image format begins with a distinctive signature - PNG starts with a specific eight-byte sequence, JPEG with 0xFFD8, GIF with the letters GIF - and those bytes are what software actually uses to decide how to open a file. The extension is only a label, and anyone can change it. If the two disagree, this page says so, because some upload forms check the extension and will refuse a file whose name does not match.

Because that particular file does not record one. Most images have no density information at all - it is only written when a camera, scanner or design tool puts it there. Many tools display 72 DPI regardless, which is a made-up number: it is a historical default, not a property of your file. Since somebody might size a print from it, this page would rather say nothing than say something invented.

No, and deliberately. A trustworthy quality score needs the decoded pixels and a defensible threshold for what counts as blurry or over-compressed, and every simple version of that is a number dressed up as a measurement. What you get instead are facts the file states about itself - its dimensions, how it was encoded, whether it is progressive, how large it is - which are the things you can actually act on.

No. The file is read in your browser and examined there. It is never decoded into pixels either, let alone sent anywhere - the page only reads the file's headers and metadata blocks, which is why it is fast on very large images and why it can describe formats your browser cannot display.

Format, size and metadata are read for JPG, PNG, WebP, GIF, BMP, AVIF, HEIC and TIFF. The deeper structure - bit depth, colour model, transparency, interlacing, density - is read for JPG, PNG, GIF, WebP and BMP. For AVIF, HEIC and TIFF the page says the structure was not read rather than guessing at it.