Fix "The archive is either in unknown format or damaged" Error

Error message: The archive is either in unknown format or damaged

Fix “The archive is either in unknown format or damaged” Error

The archive is either in unknown format or damaged is the catch-all error 7-Zip and WinRAR show when they can’t read the file’s headers. The cause is almost never what the message implies — most of the time the file format is fine and the file isn’t really “damaged” in a deep sense. Something simpler is wrong: the download finished short, the file extension misrepresents the actual format, or one part of a multi-volume set is missing. This guide walks the diagnostics in order of how often each one is actually the answer.

Quick fix

Before reaching for a repair tool, check the obvious causes. They resolve most cases in under two minutes:

Compare the file size to the source. If the download came from a website, compare the size to what the source page lists. A noticeable shortfall — even a few percent — means the download is truncated. Re-download from the original source, ideally with a download manager that supports resume rather than a browser.
Try the other tool. If 7-Zip rejected the file, try WinRAR. If WinRAR rejected it, try 7-Zip. Their tolerance for non-standard or partially damaged archives differs, and one will often open what the other won’t.
Confirm the file is what its extension claims. On macOS or Linux, run file archive.zip in a terminal — the output identifies the actual format. On Windows, drag the file into a hex viewer (Notepad++ with the HEX-Editor plugin works) and check the first few bytes: PK for ZIP, Rar! for RAR, 7z (followed by binary) for 7-Zip. If the bytes don’t match the extension, rename the file to its real format and reopen.

If size, tool choice, and format all check out and the error persists, move on.

If that didn’t work

For multi-volume archives, verify all parts are present. Multi-part archives are named archive.zip.001, archive.zip.002, … or archive.part1.rar, archive.part2.rar, … — and they only open when every volume is in the same folder, named correctly, with no gaps in the numbering. Open the first volume (.001 or .part1) — never any other part — and the tool stitches the rest together automatically. Missing or renamed volumes produce the unknown-format error on the volume the tool tried to read next.

If a single intermediate part is genuinely missing, 7-Zip can sometimes proceed by faking its presence: copy a same-size neighboring part to the missing filename so the total expected size lines up. The data in that volume will be wrong, but the archive will at least open, and other files in the set may extract cleanly. This works for non-solid 7z archives more reliably than for solid ones.

Use WinRAR’s repair feature. WinRAR has a built-in repair function that handles header corruption directly. Select the archive in WinRAR’s file pane, then go to Tools → Repair archive. Choose whether to treat the file as RAR or ZIP based on its real format, and pick a destination for the repaired output. WinRAR writes a new file (prefixed _reconstruct. or rebuilt., depending on version) rather than modifying the original, so there’s no risk to the source.

The repair is meaningful only in two cases: when the archive was created with recovery records (the -rr switch, or “Add recovery record” in the GUI), or when only the headers — not the data — are damaged. Without recovery records, WinRAR can rebuild a working file structure but cannot regenerate lost data. The output may open but contain truncated or partial files inside.

Try extraction with “Keep broken files” enabled. In WinRAR’s extraction dialog (Extract to...), tick Miscellaneous → Keep broken files. Without this option, WinRAR discards any file that fails its CRC check on extraction. With it enabled, partial files are kept on disk — useful when the archive contains many files and only some are damaged. The salvaged files may be incomplete but often contain enough usable data to be worthwhile.

Advanced recovery

For 7z archives specifically, the project’s official recovery documentation describes a header-reconstruction technique using a hex editor. The Start Header sits in the first 32 bytes of the file and contains the offset and length of the End Header, where the file listing lives. If the Start Header is intact but the End Header offset is wrong (because the file was truncated mid-write or padded with junk), the offset values can sometimes be recalculated by inspecting the file structure. This is a power-user technique — the 7-Zip recovery page documents it in detail, but it requires comfort with hex editing and an understanding of the 7z format. For most users, accepting partial recovery via Keep broken files is the more practical option.

If the archive is critical and none of the above worked, commercial recovery tools (DiskInternals ZIP Repair, Stellar Repair for ZIP, Recoverit) automate the same techniques and sometimes squeeze out a bit more data through pattern-matching on raw file signatures. They are not magic — they cannot recover bytes that were never present — but for an urgent one-off file, the modest licence cost is sometimes warranted. Be aware that this category of software has unusually high SEO spend and many of the recommendations you’ll find online are commercial placements rather than independent assessment.

Why this happens

The error name is misleading. The format isn’t unknown — 7-Zip and WinRAR both recognize their own formats and many others. The error fires whenever the tool can read enough of the file to start parsing but can’t find the structural markers it needs to continue. In practice that means one of three underlying conditions, in rough order of frequency:

The download or transfer finished short. Archive headers, especially for multi-volume sets and 7z files, depend on the file ending where the writer expected it to. If the last few kilobytes never arrived — common on flaky connections, interrupted browser downloads, and email attachments truncated by gateway scanning — the End Header offset points past the file’s actual end and the tool gives up. This is the single most common cause.

The file is in a format the tool didn’t expect. A .zip extension on a file that’s actually a RAR archive, a .rar extension on something that’s actually a 7z, or a misnamed installer that’s actually an ISO — all produce this error from a tool that opened the file expecting one structure and got another. The file command or a hex check identifies the real format in seconds.

A multi-volume archive is missing a part. Multi-part archives are continuous data streams chunked across files. Missing any chunk between the first and last produces the unknown-format error when the tool reaches the gap. Missing the first volume produces it immediately; missing later volumes produces it partway through extraction.

Real format-level corruption — bytes inside the file altered or scrambled by a faulty USB stick, a bad sector, or a flaky network without TCP-level integrity protection — is rarer than the message suggests. It does happen, particularly with files held on aging flash media, but it’s the exception rather than the rule.

Preventing this in future

Verify checksums when downloading. Many open-source projects publish SHA-256 or MD5 hashes for their release files. A two-line check (compute the hash, compare to the published value) catches truncation and transit damage before you waste time trying to extract.

For important transfers, ask the sender to add recovery records. WinRAR’s Add recovery record option (or the -rr5p CLI switch for a 5% recovery margin) increases the archive size slightly but lets WinRAR repair small areas of damage automatically. The cost is trivial; the payoff when something goes wrong is significant.

Use a download manager for files over a few hundred megabytes. Browsers do not always handle interrupted downloads gracefully, and a “completed” browser download can still be silently truncated. A dedicated download manager with resume support is more reliable for large files.

Avoid storing important archives long-term on USB sticks, especially old or cheap ones. Flash memory degrades over time, and a file that copied off cleanly six months ago may copy with bit errors today. Move important archives to a maintained drive or cloud storage as soon as you have a stable copy.

If your archive is showing a slightly different error wording — “Cannot open file as archive” rather than “unknown format or damaged” — see the cannot-open-file-as-archive guide, which covers the same family of failures from a different angle. If extraction starts but stops partway with a CRC error, the problem is data corruption inside the archive rather than a structural one; see extraction fails partway. For a broader view of how archive formats work and which tools handle which problems, the archive repair complete guide is the category pillar.

Last verified: April 2026