Convert CSV to XLSX — Free Online Spreadsheet Converter

CSV (Comma-Separated Values) is the lingua franca of data exchange — databases export it, APIs return it, analytics platforms generate it, and spreadsheet users trade it. But CSV is just raw text: no formatting, no formulas, no charts, no data types, no multiple sheets. Every cell is a string, every delimiter is ambiguous, and every opening in Excel risks mangled dates and truncated leading zeros.

Converting CSV to XLSX transforms flat text data into a proper Excel workbook with correctly typed columns, preserved number formats, and the full range of spreadsheet features unlocked. Our converter uses LibreOffice's Calc engine to intelligently parse the CSV — detecting delimiters (comma, semicolon, tab, pipe), recognizing date formats, preserving leading zeros in ID columns, and handling quoted fields with embedded commas and newlines.

The resulting XLSX file opens perfectly in Microsoft Excel, Google Sheets, LibreOffice Calc, Apple Numbers, and every modern spreadsheet application. Columns are auto-sized, data types are correctly assigned, and the file is ready for formulas, pivot tables, charts, and collaborative editing.

Opening a CSV file directly in Excel is risky. Excel's auto-detection routinely destroys data: ZIP codes like "01234" become 1234, gene names like "SEPT2" become September 2nd, phone numbers lose leading plus signs, and long numbers get rendered in scientific notation. Converting through a proper parser with explicit type handling avoids all of these data corruption traps.

XLSX offers capabilities that CSV fundamentally cannot provide. Multiple sheets let you organize related data tables in one file. Cell formatting (bold headers, colored ranges, number formats) makes data presentable for reports. Formulas and functions enable live calculations. Data validation, conditional formatting, and named ranges turn raw data into interactive tools. None of this is possible in CSV.

Collaboration workflows require XLSX. Google Sheets, SharePoint, and Microsoft Teams all understand XLSX natively for real-time co-editing. CSV files uploaded to these platforms must be converted first, often losing data in the process. Starting with a properly formatted XLSX eliminates conversion errors at the platform level.

LibreOffice Calc parses the CSV using configurable rules: delimiter detection (comma, semicolon, tab, pipe — auto-detected or user-specified), text qualifier (double quotes or single quotes), character encoding (UTF-8, UTF-16, ISO-8859-1, Windows-1252 — auto-detected via BOM or heuristics), and column type overrides. The parser handles edge cases that trip up naive implementations: fields containing the delimiter character, fields with embedded newlines, doubled escape quotes, and BOM markers.

The XLSX output uses the Office Open XML standard (ISO/IEC 29500) with deflate compression. Column widths are auto-calculated based on content length. The first row can be optionally formatted as a header row with bold text and a filter row. Numeric columns preserve full precision (up to 15 significant digits), and text columns preserve leading zeros and special characters exactly.