> Source: https://txtfetch.com/glossary/magic-bytes
> Plain-text twin — every page on txtfetch.com has one. https://txtfetch.com/text

---

# Magic bytes

The fixed byte sequence at the start of a file that identifies its real format: %PDF for a PDF, PK for a zip. It works regardless of what the extension claims.

definition

Magic bytes (or a file signature) are a fixed sequence of bytes, usually at the start of a file. They reliably identify a file's format, independent of filename or metadata.

Also called: file signature, magic number

in-plain-terms

A PDF always begins with the four bytes %PDF. A zip-based format begins with the zip signature PK, and this covers DOCX, XLSX, PPTX, and ODF alike, since they're all zip archives underneath. A PNG begins with an eight-byte fixed signature. Reading these first few bytes reliably identifies a file's real container format before any content is parsed.

Detection based on magic bytes is why an extraction pipeline can catch a file that's been renamed, mislabeled, or served with a wrong Content-Type. The bytes at the front of the file don't change no matter what the filename says. A .pdf extension on a file that's actually a renamed image gets treated as an image. It doesn't get force-fed to a PDF parser that would fail on it.

why-it-matters

-   Extension-trust detection breaks the moment a file is renamed. It also breaks when a redirect changes the name, or a source doesn't set an accurate Content-Type. Magic-byte detection doesn't depend on any of that.
-   Two very different formats can share the same magic bytes at the container level. Telling a DOCX from an XLSX needs a further look inside the archive's own manifest, not just the first four bytes.

how-to-check

-   Drop a file into the file type inspector and see exactly which bytes it read to identify the format. [Inspect a file's magic bytes](https://txtfetch.com/formats)

related-terms

-   [Media type (MIME type) →](https://txtfetch.com/glossary/media-type)
-   [OOXML (Office Open XML) →](https://txtfetch.com/glossary/ooxml)

faq

**Can two different formats share the same magic bytes?**: Yes. DOCX, XLSX, PPTX, and ODF are all zip archives underneath, so they all start with the same zip signature. Telling them apart needs a further look inside the archive's manifest, not just the first few bytes.

**Does renaming a file's extension change its magic bytes?**: No. Magic bytes live in the file's actual content, at the very start of it, and renaming only changes the filename. That's exactly why byte-level detection catches a mislabeled file that extension-based detection would miss.

related-reading

-   [How txtfetch detects a file's real type →](https://txtfetch.com/how-it-works)
-   [All glossary terms →](https://txtfetch.com/glossary)

File identity

-   [Media type (MIME type) →](https://txtfetch.com/glossary/media-type)
-   [OOXML (Office Open XML) →](https://txtfetch.com/glossary/ooxml)
-   [Document metadata →](https://txtfetch.com/glossary/document-metadata)
-   [Caption track →](https://txtfetch.com/glossary/caption-track)
-   [All terms →](https://txtfetch.com/glossary)

## See the term in real output.

Drop a file into the free reader and watch it happen.

[Open the file reader →](https://txtfetch.com/tools/file-to-text)

[Get an API key →](https://app.txtfetch.com/signup)