File extension

.xlsx

Office Open XML Spreadsheet (XLSX)

.xlsx files are Office Open XML spreadsheets: the modern Microsoft Excel format, a ZIP package (OPC) of XML parts describing worksheets, cells, formulas, styles, and charts.

Known meanings

Formats and reference material for .xlsx

Office Open XML Spreadsheet (XLSX)

ZIP-packaged XML spreadsheet format (Microsoft Excel / ECMA-376 / ISO 29500)

  • spreadsheets
  • Microsoft / ECMA / ISO
Open link

Details

Format notes

Office Open XML Spreadsheet (XLSX)

XLSX is the default workbook format of Microsoft Excel since Excel 2007 and the successor to the legacy binary .xls. Like DOCX, it is part of Office Open XML (ECMA-376 / ISO/IEC 29500) and is structurally unrelated to the old binary BIFF format it replaced: an .xlsx is an open, inspectable package of XML rather than an opaque OLE compound file.

An XLSX is a ZIP archive following the Open Packaging Conventions. Inside, each worksheet is an XML part (for example xl/worksheets/sheet1.xml), the workbook structure is in xl/workbook.xml, and shared resources are factored out into separate parts: a shared-strings table deduplicates text across cells, a styles part holds number formats and cell formatting, and further parts hold charts, drawings, and embedded media, tied together by .rels relationship files. This design keeps files reasonably compact and lets tools read or generate spreadsheets without Excel.

Interoperability And Preservation Notes

Because OOXML is an open standard supported by Excel, LibreOffice, Google Sheets, and many libraries, XLSX is a strong interchange and preservation format for spreadsheets that need to retain formulas and structure; CSV remains the choice for pure tabular data without formatting. The media type is application/vnd.openxmlformats-officedocument.spreadsheetml.sheet.

Security Notes

Spreadsheets can carry VBA macros (the .xlsm variant), external data connections, and links, so untrusted workbooks should be opened with macros disabled and external content blocked; formula-injection risks apply when building sheets from untrusted input. As a ZIP container, apply the usual precautions against decompression bombs and path traversal when extracting parts.

Further Reading

  • Office Open XML overview: https://en.wikipedia.org/wiki/Office_Open_XML