What is the GCZ file format?
.gcz is a compressed disc image format used by the Dolphin Emulator for GameCube and Wii games. It wraps a raw ISO disc image in a proprietary block-compressed container, reducing storage requirements for GameCube titles.
The format splits the disc image into 16 KiB chunks, each compressed independently with DEFLATE (zlib). Chunks that resist compression are stored raw. A block-map table in the header enables random access to any chunk, so Dolphin can decompress only the data it needs during emulation without unpacking the whole image first.
GCZ is identified by the magic number 0xB10BC00E at byte offset 0. A per-block hash table provides data integrity verification. Compression is lossless - a .gcz file can be converted back to a standard ISO at any time using Dolphin's built-in conversion tool.
GCZ is most effective with GameCube images, whose unencrypted disc padding compresses to near-zero. Wii disc images are encrypted and compress very poorly under GCZ - for Wii titles, the newer RVZ format (introduced ~2020) is strongly preferred, offering better compression ratios, deduplication, and full Wii image support.
Dolphin supports .gcz on Windows, macOS, Linux, and Android.
Security & safety
RISK: MEDIUMGCZ files themselves are compressed disc images and do not execute code - they are data containers. However, emulating GameCube/Wii games raises legal considerations: playing backups of games you own is a grey area in most jurisdictions; distributing or downloading commercial game images is copyright infringement. Obtain disc images only from games you legally own. Download Dolphin only from the official site (dolphin-emu.org) - many third-party "Dolphin" downloads bundle malware.
Format details
in a nutshellPrograms that open GCZ files
Windows1 app
Dolphin Emulator
Open-source Open Dolphin, add game folder or drag-and-drop the .gcz file. Dolphin decompresses on the fly during emulation. To convert: right-click game in list > Convert File. 
macOS1 app
Linux1 app
Dolphin Emulator
Open-source Available via Flatpak (flathub.org/apps/org.DolphinEmu.dolphin-emu) or distro repos. 
Android1 app
Dolphin Emulator (Android)
Open-source Add game directory in Dolphin Android; GCZ files load directly. Technical details
deep spec| Magic number | `0xB10BC00E` (bytes `C0 0E B1 0B` at offset 0) |
| Byte order | Little-endian |
| Compression algorithm | DEFLATE (zlib) per 16 KiB chunk |
| Block size | 16 KiB per independently compressed chunk |
| Random access | Block-map table enables direct seek to any chunk without full decompression |
| Integrity | Per-block hash table |
| Compression type | Lossless (reversible to ISO without data loss) |
| Typical compressed size | 50 MB - 1.4 GB (GameCube); Wii images compress poorly due to encryption |
| Platforms | Windows, macOS, Linux, Android |
| Best suited for | GameCube disc images (unencrypted, zero-padded disc data compresses well) |
| Superseded by | RVZ format (better compression, deduplication, full Wii support) |
| Released | ~2005 (early Dolphin emulator development) |
| Specification | github.com |
GCZ conversions
Community Q&A
asked by usersNo questions yet - be the first to ask about GCZ files.
