XLS to XLSX Converter

XLSX is the default spreadsheet format for Microsoft Excel since Office 2007, based on the Office Open XML (OOXML) standard published as ECMA-376 and adopted as ISO/IEC 29500. An XLSX file is a ZIP archive containing XML documents that describe workbook structure, sheet data, styles, shared strings, formulas, charts, pivot tables, and relationships between components. Each worksheet is stored as a separate XML part where cells are organized by row and column references with typed values and style indices. The XML foundation enables programmatic spreadsheet creation and manipulation using libraries like openpyxl (Python), Apache POI (Java), and ClosedXML (.NET) without requiring Excel. XLSX dramatically expanded capacity compared to XLS: over 1 million rows and 16,384 columns per sheet, enabling use cases previously impossible in the binary format. One advantage is openness and cross-platform support — the documented OOXML specification enables implementation by LibreOffice Calc, Google Sheets, Apple Numbers, and numerous specialized tools. Built-in ZIP compression is another practical strength: XLSX files are typically 50-75% smaller than equivalent XLS files, and the modular XML structure improves data recovery when files are partially corrupted. The format supports modern Excel features including structured tables, slicers, sparklines, Power Query connections, and real-time co-authoring. XLSX has become the standard format for spreadsheet data interchange across business, scientific, and government domains.