XLSX file format
Microsoft Excel Workbook (Office Open XML)
XLSX is Excel's workbook format: multiple sheets with formulas, formatting and charts, stored as zipped XML. Use CSV for plain data exchange and XLSX when structure and formulas matter.
| Property | XLSX |
|---|---|
| File extensions | .xlsx |
| MIME type | application/vnd.openxmlformats-officedocument.spreadsheetml.sheet |
| Introduced | 2007 by Microsoft (ISO/IEC 29500) |
| Type | Spreadsheet workbook |
| How it's stored | ZIP of XML files |
| Keeps formatting | Yes — formulas, formats, multiple sheets |
| Compatibility | Excel, Google Sheets, LibreOffice, Numbers |
| Best for | Calculations, data with formatting, multiple sheets |
What is an XLSX file?
XLSX is the Office Open XML spreadsheet format introduced with Excel 2007. A workbook can hold many sheets, each with cell values, formulas, number formats, merged cells and charts, all stored as XML inside a ZIP archive.
XLSX vs CSV
CSV holds only the values of one sheet as plain text — any app can read it, but formulas, formatting and extra sheets are lost. XLSX keeps everything but needs spreadsheet software. Fylo converts between them with the open-source SheetJS library.