XLSX - Office Open XML Spreadsheet

XLSX is the spreadsheet format Microsoft Excel has saved to by default since Excel 2007, replacing the binary XLS. It is the spreadsheet member of the same Office Open XML family as DOCX, published as ECMA-376 and as an ISO standard, which is why LibreOffice Calc, Google Sheets and Apple Numbers can all read and write it.

How an XLSX stores a workbook

An XLSX is a ZIP archive. Inside, xl/workbook.xml lists the sheets and named ranges, and each sheet gets its own part under xl/worksheets/ holding rows, cells, formulas and cell references.

One part is worth knowing about: xl/sharedStrings.xml, the shared string table. Rather than repeating a text value in every cell that uses it, Excel stores each distinct string once and has the cells point at it by index. That is what keeps a spreadsheet with fifty thousand repeated category labels from becoming enormous — and it is why a text value can persist in the file after the last cell using it was cleared.

Formulas are stored as text alongside a cached result, so a reader that cannot evaluate a formula can still show the last value Excel calculated. Charts, pivot table definitions, conditional formatting rules and embedded images each get their own part. The ZIP compression on all that XML is why an XLSX is typically a fraction of the size of the equivalent XLS.

What metadata an XLSX carries

The core and extended properties are the same pair as DOCX. Core gives you Title, Subject, Creator, Keywords, Description, Last Modified By, and the created and modified timestamps. Extended gives you Application and AppVersion, Company, and — for spreadsheets — a list of the sheet names and named ranges in the workbook, which reveals the structure of a file without opening it.

A spreadsheet also leaks in ways a text document does not, and this is worth pausing on. Hidden rows, hidden columns and hidden sheets are still in the file; hiding is a display setting, not a deletion. Cell comments and notes travel with the workbook. A pivot table keeps a cached copy of its source data, so a pivot summarising a confidential sheet can still contain that sheet's values after the sheet itself is deleted. External links record the full path of the workbook they point at, network shares included.

If you are about to send a spreadsheet outside your organisation, those four are the ones to check, in that order. None of them shows on screen.

When to use XLSX

Use XLSX for any working spreadsheet: it is compact, it keeps formulas live, it supports every current Excel feature, and it opens everywhere.

Use CSV instead when you only need the numbers and want a format that will still parse in twenty years — but accept that CSV keeps no formulas, no formatting and no multiple sheets. Export to PDF when you are sending a report someone should read rather than edit, which also solves the hidden-data problem in one step. Reserve XLS for genuinely old software that cannot open anything newer.