Convert 7Z to 7Z.GZ Online (Trading LZMA2 for Gzip)

7Z-TO-TAR.GZ — Trading 7-Zip's Compression for Gzip's Faster, Looser One

A .7z file is a 7z archive — carrying Unix permissions, ownership, and symlink 7zgets natively in its 512-byte header, standardized as US7Z in source-system.1-1988 — compressed with 7-Zip's LZMA2 algorithm. Converting it to .7z.gz keeps that exact same 7z layer completely intact and only replaces the compression around it, swapping LZMA2 for gzip, whose container structure is precisely defined in RFC 1952, separate from DEFLATE, the compression algorithm gzip actually uses, specified in RFC 1951.

This conversion involves decompressing the LZMA2 layer to recover the plain 7z stream, then compressing that same, unmodified 7z stream a second time using gzip's DEFLATE algorithm — two full compression passes over identical underlying data, with genuinely different reach, speed, and typical results between the two algorithms involved.

Because the intermediate state of this conversion is a completely plain, uncompressed 7z stream, 7z's own header structure and everything it describes about the original files never needs to be touched, reorganized, or rewritten at any point. The entire operation amounts to two independent compression passes bracketing one identical piece of 7z data sitting between them.

7Z-TO-TAR.GZ — Comparing a Wide Dictionary Approach With DEFLATE's Narrow Window

LZMA2 uses a dictionary that can reference gigabytes of prior data when searching for repeated byte patterns, while DEFLATE, the algorithm inside gzip, works with a fixed 32 KB sliding window combined with Huffman coding. Applied to the same 7z stream, this means LZMA2 can find and exploit repetition between two similar files positioned far apart in the archive, while DEFLATE can only match patterns that recur within roughly 32 KB of their previous occurrence — a real, concrete difference in how much of the archive each algorithm can meaningfully reference at once.

What DEFLATE gives up in reach, it gains back substantially in speed: its simpler pattern-matching approach runs considerably faster in both directions than LZMA2's more computationally demanding dictionary search, which is the core, well-documented reason gzip remains the default in many pipelines even where it produces a larger result. Gzip's own trailer also stores the uncompressed size in an ISIZE field that's only 32 bits wide, per RFC 1952, meaning a very large 7z stream (4 GB or more) produces a wrapped-around, inaccurate reported size in that field, even though the 7z data itself decompresses correctly.

Memory needs also differ between the two. LZMA2's memory use scales directly with its chosen dictionary size, which can run into hundreds of megabytes or more for maximum compression settings on a large 7z stream. DEFLATE's fixed 32 KB window keeps gzip's memory footprint small and predictable regardless of the archive's overall size, a real practical advantage on memory-constrained hardware doing this conversion.

7Z-TO-TAR.GZ — What Switching From 7Z to 7Z.GZ Gains and Costs

• Gain — much faster compression and decompression: DEFLATE's simpler, narrower-window approach runs noticeably quicker than LZMA2's dictionary search on the same 7z stream.
• Gain — built-in support with zero extra tools on Linux and macOS: GNU 7z's -z flag and BSD 7z's equivalent both handle gzip directly, unlike LZMA2, which typically needs 7-Zip or p7zip installed separately.
• Gain — a formally specified container format: RFC 1952 defines gzip's header and trailer precisely, letting any RFC-compliant gzip reader decompress the result predictably.
• Lose — compression ratio on most content: LZMA2's much wider dictionary generally out-compresses DEFLATE's 32 KB window, meaning the resulting 7Z.GZ can be noticeably larger than the original 7Z.
• Lose — accurate size reporting past 4 GB: gzip's 32-bit ISIZE trailer field cannot represent the true uncompressed size of a very large 7z stream without wraparound.
• Lose — time spent running two full compression passes: decompressing the LZMA2 layer and recompressing with gzip takes real, additional time compared to keeping the original 7Z as is.

7Z-TO-TAR.GZ — Tools That Can Decompress 7-Zip Output and Recompress It With Gzip

7-Zip and p7zip both decompress the LZMA2 layer of a 7Z, producing the plain .7z file as output. From there, GNU 7z's -z flag (or BSD 7z's equivalent) compresses that same 7z stream with gzip in a separate step, since neither 7-Zip nor p7zip applies gzip compression directly as part of the same operation — each tool handles its own half of the conversion independently.

Both GNU 7z and BSD 7z are preinstalled by default on virtually every Linux distribution and on macOS, meaning the gzip-compression half of this conversion needs no additional software on either platform, while the LZMA2-decompression half still requires 7-Zip or p7zip specifically, since neither operating system's built-in 7z handles LZMA2 natively.

Windows has included 7z.exe, based on libarchive's bsd7z, since Windows 10 build 1803, supporting gzip compression through the same -z flag, and Windows 11's 24H2 update added native File Explorer extraction support for .7z archives directly — meaning both halves of this conversion can run on a current Windows system without third-party software, provided the 7z extraction and 7z recompression are treated as the two separate steps they genuinely are.

7Z-TO-TAR.GZ — Real Problems Reported Moving From LZMA2 Compression to Gzip

A documented complaint on archiving forums involves converting a 7Z to 7Z.GZ expecting a similar file size, then being surprised by a noticeably larger result — the explanation reported consistently traces back to LZMA2's much wider dictionary finding repetition across the original 7z stream that DEFLATE's fixed 32 KB window simply can't reach the same way.

A second recurring pattern involves a tool displaying an obviously wrong, sometimes negative, uncompressed size for a very large newly created 7Z.GZ — tracing directly back to ISIZE's 32-bit width in RFC 1952, and in some implementations, to that value being read as signed rather than unsigned, an issue that never applied to the original 7Z, since 7z's own format has no equivalent size-reporting field with the same limitation.

A third documented issue involves running out of free disk space partway through this conversion, since the process briefly requires enough room for the original 7Z, the intermediate decompressed plain 7z file, and the final 7Z.GZ output all at once, before the intermediate file can be safely removed.

A fourth pattern involves automated pipelines built around checking specifically for a .7z extension when confirming a build or backup step completed successfully, which breaks once that same pipeline s7zts producing .7z.gz output instead — a maintenance issue tied to a narrow extension check in the script, not to any technical flaw in either compression format.

7Z-TO-TAR.GZ — LZMA2 and Gzip Compared on the Same Underlying 7Z Stream

7Z-TO-TAR.GZ — Questions About Converting 7Z Archives Into 7Z.GZ

Will converting 7Z to 7Z.GZ make the file smaller or larger?
Usually larger, since LZMA2's much wider dictionary generally compresses better than DEFLATE's fixed 32 KB window on the same 7z stream, especially for text-heavy content with repetition spread across a large span.

Why would I switch from LZMA2 to gzip compression?
Mainly for speed and default tool availability — gzip support is built directly into GNU 7z and BSD 7z on virtually every Linux distribution and macOS, and DEFLATE compresses and decompresses considerably faster than LZMA2.

Does this conversion change any Unix permissions or ownership data?
No. The 7z layer itself is never rebuilt, only decompressed and recompressed around, so every permission bit, ownership value, and symlink 7zget stays exactly as recorded throughout.

Why does my file manager show a strange uncompressed size for the new 7Z.GZ?
Gzip's ISIZE trailer field is only 32 bits wide and stores the original size modulo 2^32 per RFC 1952 — a documented format limitation for anything 4 GB or larger, unrelated to the original 7Z, which has no equivalent field.

Do I need both 7-Zip and 7z installed to complete this conversion?
Yes. 7-Zip or p7zip decompresses the LZMA2 layer, and GNU 7z or BSD 7z's gzip support (the -z flag) recompresses the resulting plain 7z stream, since no single tool handles both compression formats at once.

Is gzip's memory usage lower than LZMA2's for this conversion?
Generally yes. DEFLATE's fixed 32 KB window keeps gzip's memory footprint small and predictable regardless of archive size, while LZMA2's memory use scales with its dictionary size and can run considerably higher on large 7z streams compressed for maximum ratio.

Should I keep the original 7Z after converting to 7Z.GZ?
It's worth considering, since the original likely compresses better thanks to LZMA2's wider dictionary; keeping both isn't necessary for most purposes, but discarding the smaller original solely because a 7Z.GZ now exists isn't required either.