What is the BAC file format?
.bac files are proprietary backup archives created by Backup Plus, a Windows backup utility developed by Avantrix Inc. in the early 2000s. The software and format are discontinued and no longer supported.
A .bac archive stores a mirror of the backed-up files. When a backup spans multiple parts, the archive is split: the first segment uses the .bac extension, while subsequent parts are numbered sequentially as .b01, .b02, .b03, and so on. Restoring a split archive requires all parts to be present.
The internal format is proprietary binary, and no magic bytes have been publicly documented. Some versions of Backup Plus may have used a .ZIP-based container internally, meaning a single (non-split) .bac file can sometimes be opened by renaming it to .zip and extracting with 7-Zip or a similar utility. This approach does not work for split archives, which require the original Backup Plus Restore Manager.
Because Avantrix Backup Plus is discontinued, users encountering legacy .bac files may have difficulty finding working restore tools. If the rename-to-ZIP approach fails, recovery options are generally limited to archived copies of the original installer.
Security & safety
RISK: LOWBAC files are backup archives; they carry no executable risk on their own. The main risk is data loss - the format requires discontinued software to restore. Always keep an accessible copy in a current format.
Format details
in a nutshell- Revenue Certificate Backup (Irish ROS) - Created by Ireland's Revenue Online Service (ROS) to store a taxpayer's digital login certificate; not a general-purpose archive.
- Ami-Back Backup File - Backup archive created by Ami-Back software for the Commodore Amiga platform (1990s).
Programs that open BAC files
Windows2 apps
7-Zip
Open-source Try renaming .bac to .zip, then right-click → Open archive. Works only if the archive uses a ZIP container. 
Backup Plus Restore Manager
Free Use File > Restore to select the .bac archive and choose a destination folder. Technical details
deep spec| File encoding | Binary (proprietary Avantrix format) |
| Container | Proprietary; single-file archives may use a ZIP-based structure internally in some versions |
| Compression | Likely DEFLATE or proprietary algorithm; not publicly documented |
| Encryption | None documented |
| Split archive naming | First segment uses .bac; subsequent parts use sequential extensions .b01, .b02, .b03, etc. |
| Typical file size | Varies - mirrors the total size of the backed-up data |
| Operating system | Windows only |
| MIME type | application/octet-stream (no registered MIME type) |
| Restore tool | Backup Plus Restore Manager, bundled with Avantrix Backup Plus |
| Software status | Discontinued - Avantrix Backup Plus is no longer supported or available |
| Recovery workaround | Renaming a single (non-split) .bac file to .zip may allow extraction with 7-Zip or WinZip |
| Magic bytes | None publicly documented |
| Released | early 2000s |
BAC conversions
Community Q&A
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