What is the CHK file format?
The .chk extension refers to retrieved fragments of Windows files, produced by CHKDSK or ScanDisk - tools used for searching for errors in disk space. .chk files are simply previously lost fragments of data recovered during a file-system repair.
Sometimes, after a sudden loss of power or unplugging a USB device, data processing will not be completed. While scanning a volume with these tools, a damaged file system will be repaired and orphaned cluster chains collected. Part of the data will be stored in the FOUND.000 folder on the drive on which the damaged files were found, with each recovered fragment named sequentially: FILE0000.CHK, FILE0001.CHK, and so on.
Storing and erasing .chk files
After performing a scan of a hard drive with CHKDSK or ScanDisk, .chk files are stored in a system folder, e.g. FOUND.000. Usually there are no contraindications for deleting such files, as they are orphaned cluster chains whose source content may already be duplicated elsewhere or may be entirely empty.
Hint
A .chk file has no signature of its own - its first bytes are those of whatever the original file was (e.g. FF D8 FF for a JPEG image or PK for a ZIP archive). If there are any uncertainties regarding deletion, the content can first be viewed with the CHK-Mate program, which enables you to clearly identify what kind of data is contained within the file. Furthermore, other programs such as UnCHK and FileCHK can also retrieve data from .chk files. .chk files can also be opened using a hexadecimal editor - there is a chance you will be able to verify whether the file is empty. If after opening such a file you see only 00 bytes, this indicates that the file is empty and can be discarded.
Security & safety
RISK: LOWA .chk fragment is just recovered data, so the file itself is low-risk to inspect. Two cautions: (1) a recovered fragment could be part of any file, including a malicious one, so scan anything you reconstruct into an executable/Office document before opening it; (2) before deleting a FOUND.000 folder to reclaim space, make sure you're not discarding the only copy of data you still need - if all your real files are present and working, the .chk fragments are usually redundant and safe to delete. Avoid sketchy 'CHK file opener' download sites, which for obscure formats often bundle adware/PUPs; stick to UnCHK/FileCHK/CHK-Mate or a manual rename.
Format details
in a nutshellFULL NAMERecovered File Fragment (CHKDSK / ScanDisk)
DEVELOPERMicrosoftsince CHKDSK (since MS-DOS / early Windows); ScanDisk (MS-DOS 6.2 / Windows 9x)
CATEGORYMisc Files
MIME TYPEapplication/octet-stream
TYPENot a format: raw recovered data fragment from a damaged disk
This extension is also used by…
- Application checkpoint/temp files - A few programs (older FoxPro, WordPerfect, some backup tools) also use .chk for their own temp/checkpoint files, unrelated to disk recovery.
Programs that open CHK files
Windows4 apps
CHK-Mate
Freemium DIY DataRecovery's .chk recovery tool: scans the fragments and saves recovered files with detected types. (Slower; recommended only if UnCHK/FileCHK miss your types.)
Header inspection + rename (manual)
Built-in Open the .chk in a hex/text editor (or peek at the first bytes), identify the type (JPEG/PDF/ZIP…), then copy and rename it to that extension (FILE0000.CHK -> file.jpg).
UnCHK / FileCHK
Free Point it at the FOUND.000 folder; it scans the .chk files, detects common types by header, and restores the correct extensions in a batch.
TrID / file-type identifier
Free If you can't tell the type, run TrID on the .chk to identify the original format by its byte signature, then rename accordingly. 
Linux1 app
file + rename
Built-in Run file FILE0000.CHK to detect the real type, then cp it to the right extension and open with the matching program. Technical details
deep spec| Developer | Microsoft (CHKDSK); Microsoft/Symantec (ScanDisk) |
| Created by | CHKDSK (Windows NT through Windows 11) or ScanDisk (MS-DOS 6.2 / Windows 9x) during a disk integrity scan |
| Storage location | FOUND.000, FOUND.001, … subfolders at the root of the scanned drive |
| File naming | Sequential numeric names: FILE0000.CHK, FILE0001.CHK, … |
| Content type | Raw binary fragment - one or more orphaned cluster chains recovered from the file system |
| File signature (magic bytes) | None inherent; first bytes mirror the original file type (e.g. FF D8 FF for JPEG, 25 50 44 46 for PDF, 50 4B for ZIP/Office) |
| MIME type | application/octet-stream |
| Typical file size | One disk cluster (commonly 4 KB on NTFS) up to the full size of the lost file |
| Supported file systems | FAT16, FAT32, NTFS (CHKDSK); FAT12, FAT16, FAT32 (ScanDisk) |
| Platform | Windows only; Linux/macOS fsck does not produce .chk files |
| Recovery method | Inspect header bytes in a hex editor or run through TrID to identify the real type, then rename with the correct extension |
| Recovery tools (Windows) | CHK-Mate, UnCHK, FileCHK, TrID |
| Recovery tools (Linux/macOS) | file command plus manual rename |
| Deletion safety | Generally safe to delete; confirm first by inspecting for non-zero content in a hex editor |
| Empty file indicator | A file containing only 00 bytes recovered an empty or zeroed cluster and can safely be discarded |
| Released | CHKDSK (since MS-DOS / early Windows); ScanDisk (MS-DOS 6.2 / Windows 9x) |
| Specification | learn.microsoft.com |
CHK conversions
Community Q&A
asked by users
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Frequently asked questions
What is a .chk file?
It's a fragment of data that Windows CHKDSK or the old ScanDisk recovered from a damaged drive after a crash or improper disconnect. They sit in hidden FOUND.000 folders as FILE####.CHK and have lost their original name and extension.
How do I open a .chk file?
There's no .chk viewer - you have to find out what the file originally was. Check its first bytes in a hex/text editor (or use a tool like UnCHK/FileCHK/TrID), then rename it to the right extension (.jpg, .pdf, .docx…) and open it normally.
Can I delete .chk files and the FOUND.000 folder?
Usually yes - if all your real files are present and working, the .chk fragments are leftover orphaned data and safe to delete to free space. Only keep them if you're trying to recover something you actually lost.
Why do I have a FOUND.000 folder?
Windows created it when CHKDSK/ScanDisk found 'lost clusters' - data not linked to any file, typically after a crash, power loss, or unplugging a drive mid-write. The salvaged chunks are stored there as .chk files.
Can I recover my original files from .chk fragments?
Sometimes. A .chk that is a complete file just needs renaming to the right extension; a partial fragment may be only part of a file and not fully recoverable. Tools like UnCHK and FileCHK automate detecting and renaming the recoverable ones.
How do I convert a .chk file to JPG or PDF?
It's not a conversion - if the fragment IS a JPEG/PDF (check the header for 'FF D8 FF' or '%PDF'), just rename .chk to .jpg/.pdf. If the header doesn't match, the original wasn't that type.
References
1Microsoft - chkdsk command referencelearn.microsoft.com
2UnCHK / FileCHK (Eric Phelps)www.ericphelps.com
