What is the XLK file format?
.xlk is an automatic backup file created by Microsoft Excel when the "Always create backup" option is active in a workbook's save settings. The file is a binary-identical copy of the Excel workbook as it existed at the second-to-last save - meaning it always lags one save behind the current .xls or .xlsx file.
Excel generates or overwrites the .xlk on each save of the source spreadsheet, placing it in the same directory with the same base name but the .xlk extension. To recover data, rename the file to .xls or .xlsx and open it in Excel or a compatible application such as LibreOffice Calc.
The .xlk is not a distinct format - it mirrors the source workbook exactly. If the source was an older .xls file (Excel 97-2003), the .xlk contains an OLE2 Compound File Binary (CFBF) structure with the magic bytes D0 CF 11 E0 A1 B1 1A E1. If the source was .xlsx or .xlsm, the .xlk begins with the ZIP header (PK) and holds OOXML content. Identifying the internal format requires inspecting file content, not the extension alone.
Modern Microsoft 365 Excel relies on AutoSave and cloud version history rather than local backup files, making .xlk largely a legacy artifact encountered in older project directories.
Security & safety
RISK: LOWXLK files inherit the risk of Excel workbooks: they may contain macros (if the source was a macro-enabled workbook). However, a pure .xls/.xlsx backup renamed to .xlk is as safe as the source spreadsheet.
Format details
in a nutshellPrograms that open XLK files
Windows3 apps
Microsoft Excel
Paid Option 1: rename file from .xlk to .xlsx (or .xls), then double-click. Option 2: File → Open → change filter to 'Backup Files (*.xlk)' → select file. 
LibreOffice Calc
Open-source Rename .xlk to .xlsx or .xls, then open with LibreOffice Calc (File → Open). 
Microsoft Office (suite)
Paid Same as Excel - open via File → Open with backup filter or after rename. 
macOS2 apps
Technical details
deep spec| Internal format (XLS source) | OLE2 Compound File Binary (CFBF); magic bytes `D0 CF 11 E0 A1 B1 1A E1` |
| Internal format (XLSX source) | ZIP/OOXML (Open Packaging Convention); magic bytes `50 4B 03 04` (ZIP local file header) |
| Byte order | Little-endian |
| Compression | None for OLE2/XLS source; DEFLATE inside ZIP for XLSX/XLSM source |
| Backup timing | Always one save behind - updated each time the source workbook is saved |
| File location | Same directory as source file; same base name with `.xlk` extension |
| Recovery method | Rename to `.xls` or `.xlsx`, then open in Excel or LibreOffice Calc |
| Encryption | None (the `.xlk` itself is unencrypted) |
| Typical file size | 10 KB - 50 MB (same scale as the source workbook) |
| Platform | Windows and macOS (wherever Excel runs) |
| Activation | Enabled via File → Save As → Tools → General Options → "Always create backup" |
| Modern status | Legacy local backup; Microsoft 365 uses AutoSave and cloud version history instead |
| Released | Excel 97 era (~1997); 'Always create backup' option in Save settings |
| Latest version | N/A - extension, not a versioned format |
XLK conversions
Community Q&A
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