001 File Documentation

Summary

A split archive file (.001) is the first piece of a larger file that was cut into numbered parts (.001, .002, .003 …) so it could be emailed, uploaded, or stored on size-limited media. It is not a format of its own, so its MIME type is just application/octet-stream. You do not open the .001 alone: put every part in one folder with matching names and open the .001 with 7-Zip, which rejoins them into the original file.

Technical details

FeatureValue
Full nameSplit / multi-volume archive — first part
File extension.001 (then .002, .003, …)
MIME typeapplication/octet-stream
Format typeNot a format — a raw byte segment of another file
ContainerContainer-agnostic (parts may join into ZIP, RAR, 7z, ISO, video…)
DeveloperGeneric numbering convention (HJSplit, 7-Zip, WinRAR and others)
Magic numberNone of its own — the first part inherits the joined file’s header
Own indexNone — ordering relies on the numeric suffix, not an internal table
Rebuild ruleAll parts present, unbroken numbering, original names, same folder
Joined header: 7z37 7A BC AF 27 1C at offset 0 of the .001
Joined header: ZIP50 4B 03 04 (“PK”)
Joined header: RAR52 61 72 21 1A 07 (“Rar!”)
Typical tools7-Zip, WinRAR, PeaZip, HJSplit, Keka, The Unarchiver
Safe to deleteOnly after a verified successful join; keep every part until then
Related extensions.002, .003, .7z.001, .z01, .part1.rar, .r00

What is a .001 file?

A .001 file is the first part of a split archive: a single larger file that was divided into smaller, sequentially numbered pieces named file.001, file.002, file.003 and so on. The number is an ordering label, not a description of the contents. There is no .001 file format and no committee that owns it; the convention was popularised by the HJSplit splitter from Freebyte and is also used by 7-Zip, which writes split volumes as archive.7z.001, archive.zip.001 and the like. Its MIME type is the catch-all application/octet-stream.

Because a .001 is literally the leading byte-range of some other file, it has no structure of its own to document. What matters is how the split was made and what the parts join back into. This page explains those two things, and the one rule that decides whether the set can be rebuilt at all.

Two ways a .001 gets made

There are two distinct mechanisms behind a numbered set, and they are rejoined differently.

The first is a raw byte split. A tool such as HJSplit reads the original file and writes out fixed-size chunks with no added structure: the parts are simply the file cut at byte boundaries. To rebuild, you concatenate them back in order — nothing else. On Windows that is copy /b file.001 + file.002 + file.003 file.out; on Linux or macOS it is cat file.001 file.002 file.003 > file.out, or cat file.00* > file.out when the shell can sort them. No archiver is required because there is no archive metadata to interpret.

The second is an archiver split volume. Here 7-Zip, WinRAR or PeaZip produced the split as part of compressing an archive, writing archive.7z.001, archive.7z.002 and so on. The archiver itself understands the volume scheme, so you open the .001 in that tool and it reads the numbered siblings automatically and extracts the contents in one step. The join and the decompression happen together.

No index: the numbering is the only glue

A .001 contains no table pointing at .002 or .003. Nothing inside the first part records how many parts exist or in what order they go; the splitter relies entirely on the numeric suffix and on the parts sharing a base name. That has one hard consequence: the numbering must be unbroken and the names unchanged. If .003 is missing, or a part was renamed, or the sequence skips a number, the join fails and the missing bytes cannot be reconstructed — there is no parity or recovery data in a plain split. (RAR’s optional recovery records are a feature of the RAR container, not of the .001 numbering scheme.)

Reading the header: what will the parts become?

Because the .001 starts at byte 0 of the original file, its own first bytes are the header of whatever was split. Inspecting those bytes tells you what you are dealing with before you commit to joining.

First bytes of the .001   Joined file is
50 4B 03 04  "PK\x03\x04"  a ZIP archive
37 7A BC AF 27 1C  "7z"    a 7-Zip archive
52 61 72 21 1A 07  "Rar!"  a RAR archive
(ISO 9660 / UDF markers)   a disc image (.iso)
(container-specific)       a video, database dump, or other file

If the leading bytes are a recognisable archive signature, the joined result still needs extracting after the join. If they are something else, joining alone produces the finished file. Either way, the header settles the question of which tool to reach for, and whether a second extraction step is coming.

Rebuilding the original from the parts

The reliable procedure is the same regardless of splitter. Put every numbered part in one folder, confirm the numbering is complete and the base names match, then open the .001. 7-Zip, WinRAR and PeaZip detect the siblings and either concatenate a raw split or extract a split archive; on macOS, Keka and The Unarchiver do the same. For a raw split with no tool installed, the built-in copy /b (Windows) or cat (Unix) commands above rejoin the bytes directly. Keep all parts until you have confirmed the joined file opens correctly, then the individual .00x pieces are safe to delete.

Frequently asked questions

I only have the .001 — can I open it?

No. A .001 is just the first chunk of a bigger file. Without the remaining numbered parts the original cannot be rebuilt, and the missing data cannot be recovered from the first part alone. You need to obtain the rest of the set with its original names.

Is .001 the same as .z01 or .part1.rar?

Same idea, different convention. .z01 is a split ZIP’s first volume, .partN.rar is RAR’s modern split naming, and .001/.002 is the generic numeric scheme used by HJSplit and by 7-Zip split volumes. All three name “piece one of a set,” and all three need every piece present to rebuild.

How do I convert a .001 to .002?

You do not. A .002 is simply the next part of the same split, not a conversion target. There is no meaningful conversion of a .001 at all; the only operation is joining the parts back into the original file, which you then use or extract.

References