XLS is the legacy binary spreadsheet format from Microsoft Excel 97-2003, using the Binary Interchange File Format (BIFF8). The modern Excel format (XLSX) uses Open XML, a ZIP-compressed collection of XML files standardized as ISO/IEC 29500. Converting XLS to the modern Excel format upgrades the file from the proprietary binary structure to the open, XML-based container that current versions of Excel, Google Sheets, and LibreOffice expect.

Microsoft deprecated the XLS format with Office 2007, and modern Excel versions display a compatibility warning when opening XLS files. Features like tables, sparklines, conditional formatting rules beyond three, and worksheets with more than 65,536 rows are unavailable in the legacy format. Converting to the modern format unlocks these capabilities and removes the compatibility barrier.

Organizations maintaining archives of XLS files from the early 2000s face increasing risk as fewer applications support BIFF8 parsing. Converting to the modern format future-proofs the data while reducing file sizes by 30-50% through ZIP compression, saving storage and speeding up email attachments.

The conversion engine uses LibreOffice in headless mode to parse the XLS BIFF8 binary records, reconstruct the workbook in memory, and re-serialize it as an Open XML package. Cell values, formulas, number formats, cell styles, column widths, row heights, sheet names, and print settings are mapped to their OOXML equivalents. VBA macro code is not carried over because XLSX does not support macros — use XLSM if macros are needed.

Standard Excel formulas convert correctly. Array formulas and functions specific to very old Excel versions may need review. Dynamic array functions available in modern Excel are not affected since they did not exist in XLS.

Macros are not preserved in XLSX format because it is a macro-free format. If you need macros, save as XLSM instead. The conversion focuses on data and formatting fidelity.

Yes, usually the XLSX file is 30-50% smaller than the original XLS because the Open XML format uses ZIP compression internally.

Charts are converted to their Open XML equivalents. Most chart types transfer accurately, though some legacy 3D chart styles may look slightly different in modern Excel.

Yes, but features that exceed XLS limits — such as more than 65,536 rows or more than 256 columns — will be truncated. The reverse conversion loses any modern-only features.

Device	XLS	XLSX
Windows PC	Partial	Partial
macOS	Partial	Partial
iPhone/iPad	Partial	Partial
Android	Partial	Partial
Linux	Partial	Partial
Web Browser	No	No

Характеристика	XLS	XLSX
Полное название	Microsoft Excel 97-2003	Microsoft Excel Spreadsheet
Расширение	.xls	.xlsx
Лучше всего подходит для	Legacy support	Spreadsheet

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