zlib file format

.zz · .zlib · compression (single file) · introduced 1995 by Jean-loup Gailly & Mark Adler

What is a zlib file?

zlib is the raw DEFLATE stream format defined by RFC 1950, used inside countless other formats — PNG images, Git object storage, PDF compression streams, and HTTP "deflate" Content-Encoding all wrap zlib data. End users rarely encounter a bare .zz file because zlib is almost always embedded inside a richer container.

Most readers reach this page after double-clicking a .zz file and finding their operating system unsure what to do with it. The good news: zlib is a well-understood format with mature tooling on every major platform. It is supported in one click by mainstream archivers on Windows, macOS, and Linux.

How to extract a zlib file

Opening a .zz file is straightforward on every modern operating system. The exact steps depend on which platform you are on:

On Windows

Locate the .zz file in File Explorer.
Right-click the file. If you have 7-Zip installed, choose 7-Zip → Extract Here. On Windows 11 the built-in extractor handles ZIP, 7z, RAR and several other formats directly through Extract All…
If prompted, pick a destination folder and click Extract.
The extracted files appear in a folder next to the original archive.

On macOS

Find the .zz file in Finder.
Install The Unarchiver from the Mac App Store, or Keka for both creating and extracting.
Right-click the file → Open With → The Unarchiver (or just double-click after Unarchiver is set as the default).
The extracted folder appears next to the archive.

On Linux

Use your file manager: right-click the archive → Extract Here (file-roller on GNOME, Ark on KDE).

On the command line, run one of:

7z x archive.zz    # works for almost any format

The files appear in the current directory.

How to create a zlib file

To pack files into a .zz archive, you typically need an archiver that supports writing this format. Not every tool can create every format — for example, only WinRAR can create real RAR files. The sidebar lists the software known to read and write zlib.

On the command line, the canonical create command is:

# Use 7-Zip, PeaZip, or the format's reference tool to create .zz files.

If you regularly create archives at scale, a dedicated archive automation suite can wrap this command in scheduled jobs, integrity checks, and offsite replication.

Strengths

Universally available
Tiny code footprint
Standardized (RFC 1950)

Weaknesses and limitations

Single stream, no metadata
No filename, no permissions
Older algorithm than zstd

Typical use cases

PNG image compression
Git pack files
PDF stream compression
HTTP deflate

Technical details

zlib uses the DEFLATE algorithm and is identified on disk by the magic byte sequence 78 9C / 78 DA / 78 01. The standard MIME type is application/zlib. The format is an open specification and is free for any use.

Frequently asked questions

Is zlib safe to open?

Archive files are containers — they are only as safe as what they contain. A .zz from a trusted source is fine; one received from a stranger can hold malicious executables, scripts, or files engineered to exploit bugs in your archiver. Always keep your archiver updated, and never run unknown executables that come out of an archive. A safe workflow is to extract into a sandbox and scan the contents before opening anything.

What is the maximum file size?

Modern implementations support archives in the multi-terabyte range. The practical limit is your filesystem (FAT32 caps at 4 GB; ext4, NTFS, APFS, and exFAT all comfortably handle multi-TB files).

Can I password-protect a zlib?

Some archivers can wrap zlib files in an encrypted container, but the format itself does not always include built-in encryption. Check your archiver's documentation.

How does zlib compare to other formats?

See our format comparison pages such as ZIP vs 7z, ZIP vs RAR, and tar.gz vs tar.xz for side-by-side feature tables.