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

Trading 7-Zip's Compression for Gzip's Faster, Looser One

A .tar.7z file is a tar archive — carrying Unix permissions, ownership, and symlink targets natively in its 512-byte header, standardized as USTAR in POSIX.1-1988 — compressed with 7-Zip's LZMA2 algorithm. Converting it to .tar.gz keeps that exact same tar 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 tar stream, then compressing that same, unmodified tar 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 tar stream, tar'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 tar data sitting between them.

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 tar 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 tar stream (4 GB or more) produces a wrapped-around, inaccurate reported size in that field, even though the tar 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 tar 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.

What Switching From TAR.7Z to TAR.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 tar stream.
• Gain — built-in support with zero extra tools on Linux and macOS: GNU tar's -z flag and BSD tar'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 TAR.GZ can be noticeably larger than the original TAR.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 tar 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 TAR.7Z as is.

Tools That Can Decompress 7-Zip Output and Recompress It With Gzip

7-Zip and p7zip both decompress the LZMA2 layer of a TAR.7Z, producing the plain .tar file as output. From there, GNU tar's -z flag (or BSD tar's equivalent) compresses that same tar 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 tar and BSD tar 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 tar handles LZMA2 natively.

Windows has included tar.exe, based on libarchive's bsdtar, 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 tar recompression are treated as the two separate steps they genuinely are.

Real Problems Reported Moving From LZMA2 Compression to Gzip

A documented complaint on archiving forums involves converting a TAR.7Z to TAR.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 tar 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 TAR.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 TAR.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 TAR.7Z, the intermediate decompressed plain tar file, and the final TAR.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 starts producing .tar.gz output instead — a maintenance issue tied to a narrow extension check in the script, not to any technical flaw in either compression format.

LZMA2 and Gzip Compared on the Same Underlying TAR Stream

Questions About Converting TAR.7Z Archives Into TAR.GZ

Will converting TAR.7Z to TAR.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 tar 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 tar and BSD tar 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 tar layer itself is never rebuilt, only decompressed and recompressed around, so every permission bit, ownership value, and symlink target stays exactly as recorded throughout.

Why does my file manager show a strange uncompressed size for the new TAR.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 TAR.7Z, which has no equivalent field.

Do I need both 7-Zip and tar installed to complete this conversion?
Yes. 7-Zip or p7zip decompresses the LZMA2 layer, and GNU tar or BSD tar's gzip support (the -z flag) recompresses the resulting plain tar 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 tar streams compressed for maximum ratio.

Should I keep the original TAR.7Z after converting to TAR.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 TAR.GZ now exists isn't required either.