A ZIP of XML, not a single document
Rename an .xlsx file to .zip and you can open it: inside sit XML files for each worksheet, a shared-strings table that stores text once and references it by index, a styles part, and relationship files that tie everything together. This packaging keeps file size down and lets applications read just the parts they need. It replaced the old binary XLS, whose opaque structure was hard to interoperate with and harder to recover when corrupted.
Cells are typed rather than free text, so a number, a date, a boolean and a formula are all stored distinctly. A formula cell records both the expression and its last computed result, which is how another program can show a value without recalculating the whole sheet.
A full workbook, not just a grid
XLSX carries far more than rows and columns. A workbook holds several named sheets, each with its own cells, plus charts, conditional formatting, defined names, data validation, frozen panes and number formats that control how a stored value is displayed. Currency symbols, thousands separators and date masks are presentation applied over the underlying number, not part of the value itself.
That richness is the point of the format and also the reason it does not survive a trip to a flat file. Exporting to CSV or JSON keeps the values but discards charts, styles and the multi-sheet structure, because those plain formats have nowhere to put them.
Strengths
- Holds an entire workbook — multiple sheets, formulas, charts and formatting in one file.
- An open ISO/IEC standard read by Excel, Google Sheets and LibreOffice Calc.
- Typed cells preserve the distinction between numbers, dates, text and formulas.
- ZIP-based packaging keeps files reasonably small and resilient.
Limitations
- Far more complex than a flat file, so programmatic generation is fiddly.
- Formatting and formulas are lost when exporting to CSV or JSON.
- Large workbooks with many formulas can be slow to open and recalculate.
- Subtle differences between Excel and other apps can shift formula or date behaviour.
When to use Excel (XLSX)
Use XLSX when you need a full spreadsheet — several sheets, live formulas, charts or formatting — that others will open in Excel or a compatible app. When you only need the raw values for another system, export to CSV or JSON instead.
Excel (XLSX) FAQ
- What is the difference between XLSX and CSV?
- XLSX is a complete workbook with multiple sheets, typed cells, formulas and formatting, whereas CSV is a single plain-text grid of values with no formulas or styling. Converting XLSX to CSV keeps the numbers but drops everything else.
- How do I convert XLSX to CSV or JSON?
- Morphr converts it in your browser without uploading the file. It exports the values from the first sheet, formulas become their computed results, and formatting is dropped, leaving clean data for another tool.
- Can I open an XLSX file without Excel?
- Yes — Google Sheets and LibreOffice Calc open XLSX files directly, as do many other spreadsheet tools, since the format is a published ISO standard rather than a proprietary secret.