CSV vs XLSX: Which Should You Use?

XLSX is a full Excel workbook with formulas, formatting and multiple sheets. CSV is plain text holding one table of raw values. Use XLSX while people work on the data, and CSV when the data moves into another system.

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The short answer

A CSV file is not an Excel file, even though Excel opens it. CSV is plain text: one row per line, values separated by commas or semicolons. XLSX is Excel's own format: a compressed workbook that stores formulas, formatting, data types, charts and multiple sheets. Choose XLSX for working documents and CSV for exchanging data between programs.

The differences that matter

Formulas: XLSX keeps them; CSV stores only the calculated values.
Formatting: XLSX keeps fonts, colors, column widths and number formats; CSV has none.
Sheets: XLSX holds many sheets; CSV holds exactly one table.
Data types: XLSX knows a cell is a date, a currency or text; CSV is all text and the opening program guesses, which is how leading zeros and dates get mangled.
Compatibility: every database, programming language and import form reads CSV; XLSX needs a spreadsheet library or app.
Size: for plain data, CSV is usually smaller and much faster to process; XLSX is compressed but carries more structure.
Row limit: an Excel sheet stops at 1,048,576 rows. CSV has no limit of its own, but Excel will only show that many rows when opening one.

When to use CSV

Importing products, contacts or transactions into a store, CRM, email tool or bank system.
Exporting data for a database, a script or a data-analysis tool.
Very large datasets that would be slow or impossible to open as a workbook.
Any time two programs need to share a table and you're not sure what else they support.

When to use XLSX

Budgets, reports and models that use formulas.
Files people will read, filter, format and comment on.
Workbooks with several related sheets or charts.
Data with leading zeros, long ID numbers or dates that must not be reinterpreted.

Is CSV the same as Excel?

No. Excel can open and save CSV files, which is why they often show an Excel icon, but saving a workbook as CSV keeps only the values of the current sheet. Formulas, formatting and the other sheets are dropped. Keep the XLSX as your master copy and export a CSV whenever another system needs one.

Converting between them

Filembly converts in both directions for free, without signup. XLSX to CSV exports the first sheet as a comma-separated UTF-8 file that keeps accents intact. CSV to XLSX detects commas, semicolons, tabs and pipes automatically, so the workbook opens with every value in its own column. Files are processed in memory and never stored.