Convert XLS to XLSX — Free Online Converter

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.

This conversion expands the technical limits of the workbook significantly. XLS supports a maximum of 65,536 rows and 256 columns per sheet, while XLSX supports over one million rows and 16,384 columns. For organizations with growing datasets that have outgrown the XLS row limit, this upgrade is not optional — it is necessary to continue working with the data.

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an XLS file in modern Excel triggers a compatibility mode warning that limits features and reminds users the format is outdated.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.