7Z.001 File Documentation

Summary

A .7z.001 file is the first volume of a 7-Zip Split Archive, a single .7z archive that was cut into numbered pieces (.7z.001, .7z.002, .7z.003, ...) so it could be emailed, uploaded or stored on size-limited media. You do not open it alone: put every part in one folder with their names intact and open the .7z.001 in 7-Zip, which pulls in the other volumes and extracts the original. Its MIME type is application/x-7z-compressed. If any part is missing, the set cannot be rebuilt.

Technical details

FeatureValue
Full name7-Zip Split Archive — First Volume
File extension.7z.001
MIME typeapplication/x-7z-compressed
Format typeFirst volume of a byte-split .7z archive
DeveloperIgor Pavlov (7-Zip)
CategoryCompressed Files
Open standardYes — 7z format is open and documented
Underlying archiveOrdinary .7z (LZMA / LZMA2, optional AES-256)
Magic number (hex)37 7A BC AF 27 1C at offset 0 of the .001
Volume naming.7z.001, .7z.002, .7z.003, ... (unbroken)
Index location7z end-header sits in the LAST volume, not the first
Which part to openThe first (.7z.001) — unlike split ZIP
Self-extractable aloneNo — every volume must be present
Windows tools7-Zip, NanaZip, PeaZip, WinRAR
macOS toolsKeka, The Unarchiver
Linux / Android7-Zip / p7zip, PeaZip; ZArchiver (Android)
Related extensions.7z, .001, .z01, .zip, .rar
Specification7-zip.org/7z.html
File signature (magic bytes)
37 7A BC AF 27 1C

Offset 0 of the .7z.001 only. In ASCII the first two bytes read 7z, followed by BC AF 27 1C. This is the standard 6-byte 7z signature, and it appears at the start of the first volume because 7-Zip volume splitting is a plain byte split of the finished .7z file: the .001 holds the leading bytes (the signature header and the start of the packed streams). The later volumes (.7z.002, .7z.003, ...) are raw byte ranges with no signature of their own, and the 7z end-header (the index) sits in the LAST volume — which is why the .001 cannot be extracted alone.

What is a .7z.001 file?

A .7z.001 file is not a separate format. It is the first volume of a single 7-Zip archive that was cut into smaller, sequentially numbered pieces: archive.7z.001, archive.7z.002, archive.7z.003, and so on. 7-Zip is the open-source archiver written by Igor Pavlov, and this multi-volume splitting exists to slip a large archive under an email or upload size limit, to fit it onto size-limited media, or to make a big download resumable. The number is the order of the pieces, nothing more.

Joined back together, the volumes are an ordinary .7z archive: LZMA or LZMA2 compressed, optionally AES-256 encrypted. The reason you cannot just double-click a .7z.001 and extract it is structural, and understanding where the archive’s pieces live explains both how to open it and why a missing part is fatal.

How 7-Zip splitting works: a dumb byte split

7-Zip creates volumes by a plain byte split of the completed .7z file. It builds the whole archive in memory or on disk, then slices the finished bytes into fixed-size chunks and writes them out as .001, .002, .003. The split happens at arbitrary byte boundaries, with no regard for the archive’s internal structure. That single fact explains everything about the format:

[ complete archive.7z bytes ]
  |----- .001 -----|----- .002 -----|----- .003 -----|
   ^ 37 7A BC AF 27 1C              (raw bytes)   end-header/index ^
   signature + start of              middle        the index that
   packed streams                    of stream     lists every file

The .001 gets the leading bytes, so it carries the 7z signature 37 7A BC AF 27 1C at offset 0 and the start of the compressed data. The final volume gets the trailing bytes, which include the 7z end-header: the index that records where each file inside the archive lives. The middle volumes are just raw byte ranges with no markers at all.

Why you open the first volume, and why the last one matters

To extract, place all volumes in the same folder with their original names and open the first one, the .7z.001. 7-Zip, PeaZip and WinRAR detect the .002, .003 siblings automatically from the numbering and join them transparently; there is no separate merge step. This is the opposite of a split ZIP, where you open the final .zip part. The reason for the difference is where each format puts its index: 7-Zip’s index is in the last volume but the archive still starts at the first, and the extractor reads from the beginning, so you point it at the .001.

The end-header living in the last volume is also why opening the .001 by itself appears to “start” and then fails. The extractor reads the signature, begins decompressing, and then cannot find the index that tells it which files exist and where they end. Without the final volume the archive has no table of contents, so nothing can be safely extracted.

Unbroken numbering and unrecoverable gaps

The most common real-world problem with a .7z.001 set is a missing or renamed part. The numbering must be continuous: .001, .002, .003 with none skipped, and the base names must match. Because splitting is a raw byte split, a lost volume means a lost byte range in the middle of the compressed stream, and those bytes cannot be reconstructed from the others. There is no parity or redundancy in a plain 7z split, so an incomplete set simply cannot be rebuilt. If a download left you with .001 and .003 but no .002, you must obtain the missing piece; nothing local can regenerate it.

Joining is not converting

Databases sometimes list a 7z.001 → 7z “conversion”, but that is misleading. Putting all volumes in one folder and opening the .7z.001 in 7-Zip joins them; the result is the original .7z archive, which then extracts to its files. No transcoding happens. If you specifically need a single joined file rather than extracted contents, you can concatenate the volumes yourself (copy /b archive.7z.001 + archive.7z.002 + archive.7z.003 archive.7z on Windows, or cat archive.7z.00* > archive.7z on macOS and Linux) and then open that .7z. It is unnecessary, though, since 7-Zip joins and extracts the volumes for you when you open the first part directly. Converting to ZIP or RAR is a two-step job: extract the joined 7z, then re-archive its contents in the target format.

What is inside, and how safe it is

A split 7z set is only as safe as whatever was compressed into it, and you usually cannot see the real file types until after you have joined and extracted the set. So treat an unexpected .7z.001 the way you would any download: scan the extracted contents before running anything. The .7z.001-specific gotcha is completeness rather than security, as described above. One security note does apply: 7z volumes can be AES-256 encrypted, and password-protected archives are sometimes used to slip past antivirus scanning, so be wary of a password-protected split set that arrives unsolicited. Prefer 7-Zip, PeaZip or Keka from their official sites over “file joiner” download portals that bundle adware.

Frequently asked questions

How do I open a .7z.001 file?

Put every part (.7z.001, .7z.002, .7z.003, ...) in one folder with their original names, then open the .7z.001 with 7-Zip on Windows or Linux, or Keka or The Unarchiver on macOS. The tool detects the other volumes automatically, joins them, and extracts the original archive. There is no separate merge step.

I only have the .7z.001 — can I extract it?

No. The .7z.001 is the first slice of the archive and lacks the 7z end-header (the index), which lives in the final volume. Without every part the archive cannot be rebuilt, and because splitting is a raw byte split with no redundancy, the missing bytes cannot be recovered. You need the rest of the set.

Which part do I open — the .001 or the last one?

For a 7z volume split you open the first part, the .7z.001; 7-Zip finds the .002 and .003 siblings from it. This differs from a split ZIP, where you open the final .zip part instead. The difference comes down to where each format stores the archive’s starting point and index.

References