What is a XLSX file?

XLSX — Microsoft Excel (Office Open XML)

What is it?

XLSX has been Excel's spreadsheet format since 2007. It holds far more than numbers: formulas, conditional formatting, charts, multiple sheets, named ranges, and pivot tables.

That's the key difference with CSV: a CSV file stores only values, while XLSX stores an entire workbook, calculation logic included. Technically, a XLSX file is an archive of XML files.

History & origin

Excel has dominated the spreadsheet market since the late 1980s, relying on the binary .xls format for nearly twenty years.

XLSX arrived with Office 2007, as part of Office Open XML (ISO/IEC 29500). It also did away with its predecessor's limits: up to 1,048,576 rows and 16,384 columns per sheet, compared to just 65,536 rows before.

Why it is useful

It preserves the workbook's intelligence: change one cell, and everything recalculates. No flat exchange format can do that.

It handles multiple sheets, charts, and formatting, and remains readable in LibreOffice Calc, Google Sheets, and Numbers.

Limits & drawbacks

It's cumbersome to work with for large datasets, where a CSV file can be processed by a script in a matter of seconds.

Two well-known pitfalls: dates, which Excel is happy to reinterpret its own way, and numeric identifiers starting with a zero, which it silently strips. Lastly, a XLSX file cannot contain macros—that's what the .xlsm extension is for.

Practical tips

To share data meant for machine processing, convert to CSV: you'll lose formulas and formatting, but you'll get a lightweight, universal file. Remember to check the delimiter and encoding after conversion.

To send a table meant to be read—a budget, a report—PDF is the better choice: the layout stays fixed, and no one will accidentally alter your figures.

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