CSV to XLSX
Turn CSV into an Excel .xlsx workbook in your browser — no upload. Delimiter detected, numbers typed, leading zeros kept as text, bold header optional.
A one-sheet .xlsx with numeric and text cells classified (and counted), a preview of the first rows and a note on any ragged rows — ready to open in Excel or Sheets.
Example: A parts list with columns Name, Qty, Code, Price: Qty and Price become numbers, Code "007" stays text so its zeros survive, and parts.xlsx opens with a bold header row.
A real workbook,
written by hand.
How the CSV is parsed, how each cell is typed, and what the generated .xlsx contains.
Parsing
RFC 4180: the delimiter is detected from the first lines (comma, semicolon, tab or pipe) unless you choose one; quoted fields may contain the delimiter, line breaks and doubled quotes; a UTF-8 byte-order mark is stripped. Rows shorter than the widest are padded with empty cells and counted as ragged.
Cell typing
With numbers on, a cell that is a plain decimal (12, -3.5, .5, 1e3) becomes a numeric cell; anything else — including 007, 1,000 and dates — stays text, so leading zeros and formats you did not ask to interpret survive. Empty fields become empty cells. The header row is bold when chosen; no other formatting, formulas, column widths or merges are written.
The file
The .xlsx is SpreadsheetML (ECMA-376) written in-house: a content-types part, the workbook, one worksheet with inline strings, and a minimal style sheet, packed into a ZIP in your browser. It opens in Excel, Numbers, LibreOffice and Google Sheets. Sheet names are trimmed to 31 characters and forbidden characters replaced. Nothing leaves the browser: the file is read into memory on your device, rewritten there and offered back as a download; the same four anonymous usage counts as the rest of the site apply.
SOURCES
- ECMA-376 Part 1, 5th ed. — Office Open XML SpreadsheetML (sheet data §18.3, number formats §18.8.30, dates §18.17.4)
- RFC 4180 — Common Format and MIME Type for CSV Files (IETF, 2005)
Last reviewed 20 September 2026. How results are checked: How we verify.