What is the ZIM file format?
.zim is the file format of the openZIM project - a compressed, indexed archive designed to package an entire website for fully offline reading. The name stands for *Zeno IMproved*, succeeding the older Zeno format around 2008-2009 under the Wikimedia Foundation's Swiss entity and the Kiwix team.
The most common use is distributing offline copies of Wikipedia and other Wikimedia projects: a single .zim file may contain millions of articles, images, and associated CSS and JavaScript, compressed with ZSTD (modern) or LZMA2/xz (older) into clusters for space efficiency. A full Wikipedia snapshot with images can exceed 100 GB. The format is identified by a 4-byte magic number (ZIM\x04) at the start of the file, followed by a UUID, article counts, and pointers to the URL and title indexes.
The reference reader is Kiwix, available for Windows, macOS, Linux, Android, and iOS. kiwix-serve turns a .zim file into a local web server accessible from any browser on the network. .zim content is freely downloadable from the Kiwix Library at library.kiwix.org.
Despite sharing letters with ZIP, a .zim file is not a ZIP archive and cannot be opened as one.
Security & safety
RISK: LOWA ZIM is passive, read-only content data (HTML/images) opened in a sandboxed reader, not executable code, so it is low risk. Practical cautions: files are huge - full Wikipedia is 100+ GB, so check free disk space; and download ZIMs only from the official Kiwix Library or known mirrors to be sure of authenticity and integrity (the embedded MD5 helps verify the download isn't corrupt). It is not a ZIP, so don't try to extract it with archive tools.
Format details
in a nutshellPrograms that open ZIM files
Windows1 app
Kiwix
Open-source Install Kiwix (or Kiwix JS), File > Open the .zim (or add it to your library); browse and search offline. 
macOS1 app
Kiwix
Open-source Open the .zim in the Kiwix macOS app to read and full-text search the archive offline. 
Linux1 app
Kiwix (desktop / kiwix-serve)
Open-source Open in Kiwix Desktop, or run kiwix-serve --port 8080 file.zim to read it in a browser / share on the LAN. 
Android1 app
Kiwix for Android
Open-source Install Kiwix, download or import the .zim, and read it fully offline on the phone/tablet.
Kiwix for iOS
Open-source Install Kiwix from the App Store, add the .zim via Files/download, and read offline.
kiwix-serve / Kiwix Library
Open-source Browse hosted ZIMs online at the Kiwix Library, or self-host a .zim with kiwix-serve to read it in any browser. Technical details
deep spec| Format type | Compressed, indexed offline web-content archive (binary) |
| Developer | The openZIM project (Wikimedia CH / Kiwix) |
| Magic bytes | ZIM\x04 (0x044D495A) at offset 0 - first three bytes read ZIM with a 0x04 version marker |
| Byte order | Little-endian |
| Compression | ZSTD (modern clusters) or LZMA2/xz (older clusters); articles grouped into compressed clusters |
| Encryption | None; integrity ensured by an embedded MD5 checksum at the end of the file |
| Typical size | Tens of MB (small wikis) to 100+ GB (full Wikipedia with images) |
| Associated OS | Windows, macOS, Linux, Android, iOS (via libzim-based apps) |
| Format version | ZIM v5/v6; major version stored in the file header |
| Directory structure | Header + UUID + MIME-type list + URL/title index + compressed content clusters (HTML, CSS, JS, images) |
| Article capacity | Designed for millions of entries (e.g. full Wikipedia with all language editions) |
| Checksum | MD5 embedded at the end of the file; readers verify the archive before use |
| MIME type | application/x-zim |
| Primary reader | Kiwix (desktop, Android, iOS, kiwix-serve); all readers use the libzim library |
| Released | 2008-2009 (openZIM, successor to the Zeno format) |
| Latest version | ZIM format v6 (libzim 6+/9+; clusters with ZSTD or LZMA/xz) |
| Open standard | Yes · royalty-free |
| Specification | wiki.openzim.org |
ZIM conversions
Community Q&A
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