How is the real format found?

From the file's first bytes, not its name. A PNG renamed to .jpg is still flagged as a PNG.

How accurate is the JPEG quality?

It's an estimate that matches the file's tables against the standard ones. Encoders that use custom tables can read a little off.

Many files don't store one. DPI only matters for printing, and pixels are what count on screens.

No. Reading files happens in your browser with WebAssembly, so nothing is sent to a server.

Image size checker

See what's really in an image file, and test it against any upload limit.

Runs on your device. Nothing is uploaded.

Add files
Drop images, or click to browse. Details appear right away.
Read the panel
Copy any value with its button.
Set a limit
Enter a maximum size or dimensions to see pass or fail for each file.
Export
Copy everything as JSON, or download it.