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7z is the native archive format of 7-Zip, designed to provide the highest compression ratio using advanced compression algorithms and solid compression.

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Format Overview

The 7z format offers several key advantages:
  • High compression ratio - Typically 30-70% better than ZIP
  • Open architecture - Supports any compression method
  • Strong AES-256 encryption - Encrypts both file names and data
  • Large file support - Up to 16 exabytes (2^64 bytes)
  • Solid compression - Treats multiple files as continuous stream
  • Unicode file names - Full UTF-16 support

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Format Structure

According to the official format specification (source/DOC/7zFormat.txt), a 7z archive consists of: 
SignatureHeader
[PackedStreams]
[PackedStreamsForHeaders]
[
  Header 
  or 
  {
    Packed Header
    HeaderInfo
  }
]

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Signature Header

Every 7z file begins with a signature header: 
BYTE kSignature[6] = {'7', 'z', 0xBC, 0xAF, 0x27, 0x1C};

ArchiveVersion {
  BYTE Major;   // now = 0
  BYTE Minor;   // now = 4
};

UINT32 StartHeaderCRC;

StartHeader {
  REAL_UINT64 NextHeaderOffset
  REAL_UINT64 NextHeaderSize
  UINT32 NextHeaderCRC
}
The signature is always 37 7A BC AF 27 1C in hexadecimal.
The current format version is 0.4 (Major=0, Minor=4). All 7z archives created by 7-Zip 18.06 and later use this version.

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Compression Methods

The 7z format supports multiple compression codecs, defined in source/CPP/7zip/Archive/7z/7zHeader.h:100-128:

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Primary Compression

  • LZMA2 (0x21) - Default, best compression ratio
  • LZMA (0x30101) - Original algorithm
  • PPMd (0x30401) - Good for text files
  • Deflate (0x40108) - Compatible with ZIP
  • BZip2 (0x40202) - Alternative compression

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Filters (BCJ)

Filters improve compression for executable files:
  • BCJ (0x3030103) - x86 executables
  • BCJ2 (0x303011B) - x86 with better ratio
  • ARM (0x3030501) - ARM executables
  • ARM64 (0xa) - ARM64 executables
  • PPC (0x3030205) - PowerPC executables
  • IA64 (0x3030401) - Itanium executables
  • SPARC (0x3030805) - SPARC executables

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Encryption

  • AES-256 (0x6F10701) - Strong encryption with SHA-256 key derivation

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Usage Examples

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Create Archive with Maximum Compression

7z a -t7z -m0=lzma2 -mx=9 -mfb=64 -md=32m -ms=on archive.7z files/
Parameters:
  • -t7z - Use 7z format
  • -m0=lzma2 - Use LZMA2 compression
  • -mx=9 - Maximum compression level
  • -mfb=64 - 64 word size for fast bytes
  • -md=32m - 32 MB dictionary size
  • -ms=on - Enable solid mode

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Create Encrypted Archive

7z a -t7z -p -mhe=on secure.7z files/
Parameters:
  • -p - Prompt for password
  • -mhe=on - Encrypt headers (file names)
When using -mhe=on, both file names and data are encrypted with AES-256. Without this option, only file data is encrypted and file names remain visible.

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Create Multi-Volume Archive

7z a -t7z -v100m archive.7z files/
Creates volumes: archive.7z.001, archive.7z.002, etc., each 100 MB.

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Solid vs Non-Solid Archives

Solid archive (better compression): 
7z a -t7z -ms=on solid.7z files/
Non-solid archive (faster single-file extraction): 
7z a -t7z -ms=off nonsolid.7z files/
Solid compression treats all files as a continuous data stream, achieving better compression but requiring extraction of all prior files to access a specific file.

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Advanced Features

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Compression Method Chaining

The 7z format supports multiple compression methods in a chain: 
7z a -t7z -m0=BCJ -m1=LZMA2:d=24m executable.7z program.exe
This applies BCJ filter first (for x86 code), then LZMA2 compression.

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Self-Extracting Archives

7z a -sfx archive.exe files/
Creates a Windows executable that extracts itself.

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Archive Update

The 7z format supports efficient updates: 
7z u archive.7z newfile.txt
Only modified files are recompressed.

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File Properties

The 7z format stores comprehensive file metadata (7zFormat.txt:80-89):
  • Names (0x11) - UTF-16 file names
  • CTime (0x12) - Creation time
  • ATime (0x13) - Last access time
  • MTime (0x14) - Modification time
  • WinAttributes (0x15) - Windows file attributes
  • Comment (0x16) - File comments

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Format Limitations

While the 7z specification supports files up to 16 exabytes, practical limitations include:
  • Available RAM for dictionary during compression/decompression
  • File system limitations (FAT32, NTFS, etc.)
  • 32-bit vs 64-bit executable limitations

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Implementation Details

The 7z format handler is implemented in: 
source/CPP/7zip/Archive/7z/
├── 7zHandler.cpp      # Main archive handler
├── 7zHandlerOut.cpp   # Archive creation
├── 7zIn.cpp           # Archive reading
├── 7zOut.cpp          # Archive writing
├── 7zDecode.cpp       # Decompression
├── 7zEncode.cpp       # Compression
├── 7zHeader.h         # Format constants
└── 7zItem.h           # Item structures

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Header Structure

From source/CPP/7zip/Archive/7z/7zHeader.h:11-37: 
const unsigned kSignatureSize = 6;
extern Byte kSignature[kSignatureSize];

struct CArchiveVersion {
  Byte Major;
  Byte Minor;
};

const Byte kMajorVersion = 0;

struct CStartHeader {
  UInt64 NextHeaderOffset;
  UInt64 NextHeaderSize;
  UInt32 NextHeaderCRC;
};

const UInt32 kStartHeaderSize = 20;

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Performance Considerations

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Compression Level Impact

LevelSpeedRatioDictionaryDescription
-mx=1FastestLowest64 KBFast compression
-mx=5FastNormal16 MBDefault
-mx=7MediumGood32 MBHigh compression
-mx=9SlowBest64 MBUltra compression

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Memory Requirements

Memory usage during compression (LZMA2): 
Memory ≈ (Dictionary × 11.5) + 6 MB
For decompression: 
Memory ≈ Dictionary + 3 MB

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Comparison with Other Formats

7z vs ZIP:
  • Compression: 7z typically 30-70% better
  • Speed: ZIP faster for creation and extraction
  • Compatibility: ZIP more widely supported
  • Features: 7z has solid compression, better encryption
7z vs RAR:
  • Compression: Similar ratio, depends on content
  • Open source: 7z is open source, RAR is proprietary
  • Speed: RAR slightly faster for creation
  • Recovery: RAR has better recovery records

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Best Practices

For Distribution

Use -mx=5 or -mx=7 for good compression with reasonable speed

For Archival

Use -mx=9 -ms=on for maximum compression

For Security

Use -p -mhe=on to encrypt both data and file names

For Updates

Use non-solid archives (-ms=off) for better update performance

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See Also

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