Free online BAK to XLSX converter

An older document backup may need to be opened in a current spreadsheet program, shared as an Excel workbook, or archived in a standard editable format. The conversion succeeds only after identifying what the .bak file contains and, when necessary, restoring it with the application that created it.

What the BAK format is

.bak is a generic backup filename extension, not a standardized document format. Document editors, spreadsheet programs, accounting systems, databases, and other applications can use it for an earlier copy, recovery file, or proprietary backup.

A BAK file may contain an Excel workbook, an OpenDocument spreadsheet, another document, or application-specific data. Its filename, parent folder, timestamp, and the program that created it are the best clues to its contents. Changing .bak to .xlsx changes only the filename; it does not convert the data.

What the XLSX format is

.xlsx is the Office Open XML workbook format used by Microsoft Excel and supported by LibreOffice Calc, Apple Numbers, and other spreadsheet applications. It is a ZIP-based package containing XML files for worksheets, cell values, formulas, formatting, charts, tables, relationships, and workbook settings.

XLSX is suitable for editing and sharing tabular data with current spreadsheet software. It does not preserve every feature from another application, and it cannot represent database relationships, permissions, stored procedures, or proprietary forms. VBA macros require .xlsm; saving a macro-enabled workbook as .xlsx removes its VBA project.

How to convert a spreadsheet BAK file

1. Make a copy of the backup and work on the copy. Keep the original unchanged until the recovered workbook has been checked.
2. Identify the creating program from the original file's location, filename, backup date, documentation, or the application used to create the document.
3. Open the copy in that application through File → Open. If the program hides BAK files, choose All files in the file-type selector. Use the application's restore or recovery command when one is available.
4. If the program opens the file as a spreadsheet, inspect several worksheets, formulas, dates, charts, tables, named ranges, and external links. Choose File → Save As or File → Export, then select Excel Workbook (*.xlsx) or the equivalent Excel 2007–365 (*.xlsx) option.
5. Save to a new filename such as recovered.xlsx. Reopen that file in the target spreadsheet program and check formula results, formatting, row counts, and important text before replacing the backup.

Microsoft Excel or LibreOffice Calc is appropriate when the backup is already an Excel-compatible workbook. If the file is an older binary Excel workbook, the program may identify it after opening it as .bak or after making a copy named recovered.xls; save the opened document as .xlsx rather than relying on renaming alone. A backup of an OpenDocument spreadsheet should first be opened in LibreOffice Calc or its original program and then exported as XLSX.

To inspect a copy without changing it, run file backup.bak on macOS or Linux, or examine it with a file-archive utility. A file beginning with PK may be an XLSX ZIP package, while D0 CF 11 E0 usually indicates an older Microsoft Compound File format. These signatures are only clues: a ZIP file is not necessarily an XLSX workbook, so verify that it contains workbook package files and open it in spreadsheet software before resaving.

You can also use the free BAK-to-XLSX converter on 101convert.com without installing software when the service recognizes the backup's underlying spreadsheet format. It cannot reliably interpret a proprietary, encrypted, damaged, or non-spreadsheet backup merely from the .bak extension.

When the original application is required

If the creating program does not open the file as a workbook, use its restore, repair, import, or recovery function. After recovery, choose that program's spreadsheet export command, such as File → Export → Excel, and select XLSX if offered.

A generic converter cannot infer the structure of an application-specific backup. A backup may contain internal records, compressed data, permissions, or document objects rather than cells. If the original application is unavailable, use its documented recovery utility or obtain a compatible version; do not assume that an arbitrary BAK file can be made into a valid workbook.

Quality and compatibility limits

A backup can represent an earlier save and may omit edits made after it was created. Compare its contents and timestamp with the current document when both are available. Password-protected, encrypted, corrupted, or partially written files may require repair before export.

Conversion can alter unsupported formatting, formulas, external links, named ranges, pivot tables, charts, dates, Unicode text, and macros. Verify formula results and inspect every worksheet after saving. An XLSX worksheet is limited to 1,048,576 rows and 16,384 columns; larger datasets must be split across sheets or exported to a database or another format.