6PACK File

What are 6PACK files and how to open them

Are you having problems opening a 6PACK file or are you simply curious about its contents? We're here to explain the properties of these files and provide you with software that can open or handle your 6PACK files.

What is a 6PACK file?

A .6PACK file is a 6pack Compressed Data file.

6pack Compressed Data is a container format for compressing one or more files into a single archive, using the FastLZ compression algorithm. It is created by the 6pack utility distributed with FastLZ, by Ariya Hidayat. Its main job is to package files into a compressed archive; to unpack, one uses 6unpack.

Inside the archive, files are stored in "chunks." Each chunk has a header that indicates whether the data is stored uncompressed or compressed using FastLZ, and includes an Adler-32 checksum for data integrity, the compressed size, and the expected decompressed size. The user runs 6pack to compress files or directories; afterward, 6unpack reads the archive, verifies checksums, decompresses any compressed chunks, and writes the original files.

The file begins with an 8-byte magic signature: bytes 0-7 are 137, '6', 'P', 'K', 13, 10, 26, 10. After that, the file is organized as a sequence of chunks. Each chunk header is 16 bytes: a 2-byte chunk-id, 2-byte options flag (where one option is "compressed vs stored"), a 4-byte chunk size, a 4-byte checksum (Adler-32), and a 4-byte "extra" field giving the uncompressed size for compressed chunks.

How to open 6PACK files

Important: Different programs may use files with the 6PACK file extension for different purposes, so unless you are sure which format your 6PACK file is, you may need to try a few different programs.

We have not yet verified any programs for Windows that work with this specific file format. If you know one, please use the 'Suggest a program' link below. Thanks!

Last updated: August 16, 2026

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