What is the BC file format?
A .bc file is an incomplete download being managed by BitComet, the Windows BitTorrent client. While a download is in progress, BitComet appends .bc to the target filename - for example, movie.avi becomes movie.avi.bc - and the file grows on disk as downloaded and hash-verified pieces arrive.
The .bc file has no container structure of its own: it is the actual target file's raw bytes, with only the portions received so far written to disk. Once the download finishes and all piece hashes pass verification, BitComet strips the .bc extension to reveal the complete file. BitComet also uses the closely related .bc! variant (with an exclamation mark) as an alternate incomplete-file suffix; the two are functionally identical.
Other download clients use different markers for the same purpose: Firefox and qBittorrent use .part, Google Chrome uses .crdownload, and µTorrent uses .!ut.
Important warnings:
- Do not rename or force-open a
.bcfile while the download is still running; the data is incomplete and will produce errors or corrupt playback - Do not delete
.bcfiles for active downloads - BitComet cannot resume without the partial data - A companion sidecar (same base name,
.bc!suffix) tracks which pieces have been verified; deleting it separately may disrupt resumption
Security & safety
RISK: LOWThe .bc file itself is just partial data from a download and poses no more risk than the final file would. The real safety question is about the content being downloaded and its source. Sideloading copyrighted content via BitTorrent is illegal in many jurisdictions. Downloaded executables (games, software) should be scanned with antivirus after completion. No known malware exploits the .bc partial-file format itself.
Format details
in a nutshell- bc programming language bytecode / GNU bc arbitrary-precision calculator - POSIX-standard command-line calculator (bc) uses no file extension; not related.
- BlackBerry Curve backup file - Some BlackBerry Desktop Manager versions saved backups with .bc extension; rare today.
Programs that open BC files
Windows1 app
BitComet
Freemium If the download is in the list, simply let BitComet resume it. If lost, re-add the original .torrent file in BitComet and point it to the folder containing the .bc file - BitComet will hash-check and resume. Technical details
deep spec| Created by | BitComet BitTorrent client (Windows) |
| Format type | Raw partial binary content of the target file; no wrapper or header of its own |
| Magic bytes | None - mirrors the target file's bytes; only the .bc extension signals incompleteness |
| Piece integrity | BitTorrent SHA-1 hashes (protocol v1) or SHA-256 (v2) verified by the client against .torrent metadata |
| Companion file | .bc! sidecar tracks verified piece map; required for download resumption |
| Typical file size | 0 bytes (just started) to full target file size (nearly complete) |
| Encoding | Binary (exact mirror of target file encoding) |
| Operating system | Windows (BitComet is Windows-only) |
| Alternate variant | .bc! (exclamation mark suffix) - functionally identical to .bc |
| Analogous formats | .part (Firefox/qBittorrent), .crdownload (Chrome), .!ut (µTorrent) |
| Extension removal | BitComet strips .bc automatically when download completes and hash check passes |
| Container | None - raw downloaded data stored sequentially or sparsely to disk |
| Released | circa 2003 (BitComet initial release) |
| Latest version | N/A (internal marker, no versioned spec) |
| Specification | wiki.bitcomet.com |
BC conversions
Community Q&A
asked by usersNo questions yet - be the first to ask about BC files.
