What is the 001 file format?
A file with the .001 extension contains the first part of a data archive - or any large file - that has been split using a file-splitting or archiving tool. .001 is a generic suffix used by many archive-splitting tools, which may support various compression methods. Because the format is container-agnostic, the joined result may be a ZIP archive, a 7-Zip archive, an ISO image, a video, or any other binary file.
In order to access the original data, all subsequent parts must be present - the ascending numeric naming convention (.001, .002, .003, …) tells the joining tool in what order to concatenate them. Some splitters such as HJSplit embed lightweight control information in the first segment so the program can determine how many parts are required to recreate the file; plain binary splits carry no such metadata.
.001 files are commonly used for splitting larger archives into smaller parts that are easier to manage, transport and store - for example, on optical discs or USB drives - or for creating smaller email attachments. The format is also widely used for splitting RAR archives and ISO images into numbered segments.
Tools like 7-Zip generate a double extension such as .7z.001 or .zip.001, making the inner format explicit. Because .001 carries no magic bytes of its own, the first bytes of the file belong to whichever format was split: a split 7-Zip volume starts with the 7z header bytes, a split ZIP starts with PK, and a plain HJSplit split shows whatever header the original file had.
Security & safety
RISK: MEDIUMA split set is only as safe as whatever was split - most often an archive that can contain malware you won't see until after joining and extracting, so scan unexpected .001 sets before running anything inside. The .001-specific gotcha is completeness, not security: every part must be present with unbroken numbering (.001, .002, .003 …) and original names, or the join fails and the missing bytes cannot be recovered. Beware fake "file joiner" download portals bundling adware - prefer 7-Zip, PeaZip or HJSplit from their official sites.
Format details
in a nutshellPrograms that open 001 files
Windows5 apps
WinRAR
Paid With every part in the folder, open file.001; WinRAR joins the numbered volumes. (db website rarlab.com is fine.) 
7-Zip
Open-source Put all parts in one folder. Right-click file.001 > 7-Zip > Extract Here (for 7z/zip split volumes) or Open to join a raw split. Free; the go-to tool. 
PeaZip
Open-source Open file.001 with all parts present; PeaZip reassembles split sets. Free. (db URL peazip.org redirects to peazip.github.io.) 
HJSplit
Free The classic byte-splitter that often created .001 sets - use its Join function on the .001 to rebuild the original. 
copy /b (built-in)
Built-in Raw split only: copy /b file.001 + file.002 + file.003 file.out - concatenates parts without any tool. 
macOS3 apps
The Unarchiver
Free Handles multi-part 7z/zip/rar split sets - open the .001 with the siblings present.
Keka
Free Open file.001 with all parts in one folder; Keka joins split volumes and extracts archive sets.
cat (Terminal)
Built-in Raw split: cat file.001 file.002 file.003 > file.out in the folder to rejoin. 
Linux4 apps
Xarchiver
Open-source Lightweight archive manager (active GitHub fork) - opens split archive sets from the .001. 
File Roller
Open-source GNOME archive manager - open file.001 with the parts present. (Project moved from SourceForge to GNOME GitLab.)
7-Zip / p7zip
Open-source Run 7z x file.001 (split archive) - reads .002/.003 siblings automatically. Technical details
deep spec| Format type | First segment of a sequentially numbered file split; remaining parts use extensions .002, .003, and so on |
| MIME type | application/octet-stream |
| Magic bytes | None of its own - inherits the first bytes of the original file (e.g., 50 4B 03 04 for a split ZIP; 37 7A BC AF 27 1C for a split 7-Zip volume; 52 61 72 21 for a split RAR) |
| Compression | None intrinsic - depends entirely on whether the source file being split was itself compressed |
| Container-agnostic | Yes - the joined output may be a ZIP, RAR, 7-Zip archive, ISO image, video, or any other binary format |
| Metadata / control info | Optional; HJSplit embeds a small header storing the total part count; plain binary splits via cat or copy /b carry no metadata |
| Join command (Windows) | copy /b file.001+file.002+file.003 output.ext |
| Join command (Linux / macOS) | cat file.001 file.002 file.003 > output.ext |
| 7-Zip naming variant | 7-Zip appends .001 after the inner format extension - e.g., archive.7z.001 or archive.zip.001 - to make the container type explicit |
| Maximum numeric suffix | Conventionally up to .999 (3-digit sequence); some tools extend beyond this with additional digits |
| Split size | Set by the user at split time; common values include 700 MB (CD-ROM), 4.7 GB (single-layer DVD), and arbitrary byte counts |
| Error detection | None built into the .001 convention itself; integrity depends on the wrapped format (RAR volumes include per-segment CRC checksums; HJSplit and plain splits have no built-in checks) |
| All parts required | Yes - every segment from .001 to the last numbered part must be present and intact before the joined file can be used |
| Developer / origin | No single developer; generic convention popularised by HJSplit (Freebyte) and subsequently adopted by 7-Zip, WinRAR, PeaZip, and many other splitting tools |
| Naming convention | Ascending zero-padded integers as the file extension - .001, .002, .003, … - with all parts sharing the same base name |
| Released | Sequential .001/.002 numbering popularised by HJSplit (Freebyte) and generic file splitters; 7-Zip uses .7z.001/.zip.001 for its split volumes |
| Open standard | Yes · royalty-free |
001 conversions
Community Q&A
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