File Signature (Magic Number) Lookup

About this calculator

Most file formats begin with a fixed sequence of bytes called a magic number or file signature — for example JPEG images start with FF D8 FF, PDFs with 25 50 44 46 ("%PDF"), and ZIP-based formats (including DOCX and XLSX) with 50 4B 03 04 — that identify the format regardless of the file's extension. Because signatures live in the file's actual content rather than its name, they're the basis for tools like the Unix `file` command and are far more reliable than trusting a renamed or extension-stripped file.

This lookup takes the leading bytes of a file, entered as hex, and matches them against a reference table of common image, archive, document, and executable signatures to identify the true format. Digital forensics investigators, malware analysts, and developers building upload validators or file-type detectors use it to confirm what a file actually is — catching disguised executables, corrupted downloads, or mislabeled files that a simple extension check would miss.