Developer Tools

CSV / Excel Converter

Convert CSV files to Excel (.xlsx) or Excel files to CSV. All processing happens in your browser — no files are sent to the server.

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Tips

  • When converting Excel → CSV, the output includes a UTF-8 BOM so Excel on Windows opens it without garbled characters.
  • If the Excel file has multiple sheets, a dropdown appears so you can choose which sheet to export.
  • All file processing happens in your browser — nothing is uploaded to any server. Safe for sensitive data.
  • When converting CSV → Excel, Excel may auto-convert some values (e.g. "1-2" → a date). This is Excel behaviour, not a conversion error.

FAQ

When converting Excel → CSV, a sheet selector dropdown appears if there is more than one sheet. Only the selected sheet is exported to CSV.

The output CSV uses UTF-8 with BOM. The BOM ensures that Windows Excel opens the file without garbled characters.

The limit depends on your browser's available memory. Files up to several dozen MB typically convert without issues. Very large files may cause the browser to become unresponsive.

Side Note — Why Excel Corrupts CSV Files

The problem of Excel silently converting strings to dates has frustrated data engineers worldwide for decades. Values like 1-2 or 3/4 get interpreted as dates and cannot easily be reverted. Microsoft has been aware of this since at least the 2010s, but backward-compatibility constraints made it hard to fix; it wasn't until 2021 that an update finally allowed users to specify column types on import.

The issue was most damaging in genomics research. A 2004 study found that roughly 20% of gene name lists in published papers had been corrupted by Excel's auto-conversion. For example, the gene SEPT2 (Septin-2) would be converted to 2-Sep (September 2nd). In response, the HUGO Gene Nomenclature Committee took the extraordinary step in 2020 of renaming affected genes to avoid the problem.

The most important thing to watch when converting between CSV and Excel is character encoding. Excel on Japanese Windows saves CSVs as Shift-JIS, while Google Sheets and Excel on Mac use UTF-8. This tool outputs UTF-8 with BOM when converting Excel → CSV, so it opens correctly in Windows Excel without garbled characters.

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