What is the BKS file format?
A .bks file is a Microsoft Works Spreadsheet Backup file - an automatic backup copy of a Works Spreadsheet (.wks) file, created by the Microsoft Works Spreadsheet application. Each time a .wks file is saved, Works writes the previous version to a .bks file with the same base name in the same folder, replacing any prior backup. This mechanism provided a single-generation safety net against accidental data loss.
Structurally, a .bks file is a direct binary copy of its source .wks file in the proprietary Microsoft Works Spreadsheet format - there is no distinct container or wrapper. To recover data from a .bks file, rename the extension to .wks and open it in Works, or import it using LibreOffice Calc, which provides partial Works format support.
Microsoft Works was a low-cost integrated productivity suite bundled with many consumer Windows PCs from the late 1980s onward. The final release, Works 9 (2007), was the last version to generate .bks backup files. Microsoft discontinued Works in January 2010. Works Spreadsheet files are unrelated to Excel's .xls format, though Works could export to .xlr for Excel compatibility. The .bks extension is specific to Works Spreadsheet backups and should not be confused with the Windows NT backup catalog format sharing the same extension.
Security & safety
RISK: LOWBinary spreadsheet data file; not executable. No macro capability in Works Spreadsheet format. Safe to rename and open.
Format details
in a nutshell- BookSmart backup (Blurb) - Blurb BookSmart photo book software historically used .bks for project backup files - unrelated to Works.
- General 'backup' placeholder - Some applications use .bks as a generic backup extension when saving a copy of the primary file before modification.
Programs that open BKS files
Windows2 apps
LibreOffice Calc
Open-source Rename file.bks to file.wks, then open in LibreOffice Calc (File > Open). LibreOffice supports Works Spreadsheet import. 
Microsoft Works 9
Paid If Works 9 is installed: rename .bks to .wks and open normally. Works is EOL (2010) and no longer sold. Technical details
deep spec| Format type | Binary Microsoft Works Spreadsheet backup |
| Encoding | Binary, little-endian |
| Relationship to source | Byte-for-byte copy of the corresponding `.wks` Works Spreadsheet file; no distinct wrapper |
| Backup trigger | Created automatically each time the associated `.wks` file is saved |
| Backup depth | Single generation - each save overwrites the previous `.bks` with the then-current spreadsheet state |
| Recovery method | Rename `.bks` to `.wks`, then open in Microsoft Works or a compatible application |
| Container | Microsoft Works Spreadsheet proprietary binary format (identical structure to `.wks`) |
| Typical file size | Under 1 KB to ~5 MB (mirrors the source `.wks` file size) |
| Works versions | Works 4.x through Works 9 (final release) |
| Related formats | `.wks` (source spreadsheet), `.wps` (Works word processor), `.xlr` (Works/Excel exchange format) |
| Compatible openers | LibreOffice Calc (partial support); legacy Microsoft Works 9 |
| Integrity | No checksum; file consistency relies on the OS file system |
| Released | Late 1980s (with Microsoft Works for Windows) |
| Latest version | Microsoft Works 9 (2007, final version) |
BKS conversions
Community Q&A
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