XLS is the Binary Interchange File Format (BIFF8) used by Microsoft Excel from 1997 to 2003. XLSX is the Office Open XML format introduced with Excel 2007, storing data as ZIP-compressed XML files conforming to ISO/IEC 29500. Converting XLS to XLSX is the most common spreadsheet format migration — it upgrades legacy files to the current standard used by Excel, Google Sheets, Numbers, and every modern spreadsheet application.

Microsoft stopped developing the XLS format in 2007. Every version of Excel since then has used XLSX as the default save format. Opening an XLS file in modern Excel triggers a compatibility mode warning that limits features and reminds users the format is outdated. Converting to XLSX removes this limitation, enables modern features like dynamic arrays and Power Query, and ensures full interoperability with cloud services like OneDrive and SharePoint.

XLSX files are also substantially smaller than XLS equivalents because of ZIP compression. A 10 MB XLS file often compresses to 3-4 MB as XLSX. For organizations maintaining thousands of legacy spreadsheets, batch conversion to XLSX can recover significant storage space while simultaneously improving compatibility and data integrity.

The conversion engine uses LibreOffice in headless mode to parse the XLS BIFF8 binary records — cell values, shared strings, formatting records, formulas, and named ranges — and re-serialize the workbook as an Office Open XML SpreadsheetML package. The output XLSX conforms to the Transitional variant of ISO/IEC 29500, which provides the broadest compatibility with existing software.

Functionally, yes. The server-side conversion uses the same file format parser and writer. The advantage is batch processing without needing Excel installed.

No. XLSX is a macro-free format by design. If the XLS contains VBA macros, you need the XLSM format to preserve them. The conversion focuses on data and formatting.

Typically 30-60% smaller. XLSX uses ZIP compression internally. A 10 MB XLS file often converts to 3-5 MB as XLSX, depending on content density.

Yes. Named ranges, data validation rules, and cell comments convert to their XLSX equivalents. Very complex nested data validation may need verification after conversion.

Yes, but any data exceeding XLS limits (more than 65,536 rows or 256 columns) will be truncated. Features exclusive to XLSX will also be lost.

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

Funktion	XLS	XLSX
Fullstandigt namn	Microsoft Excel 97-2003	Microsoft Excel Spreadsheet
Filandelse	.xls	.xlsx
Bast for	Legacy support	Spreadsheet

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