Is the output a real Excel file?

Yes. The tool builds a genuine OOXML (.xlsx) workbook — a ZIP package of XML parts — with a correct CRC-32 on every entry, which is exactly what Excel produces. Excel, LibreOffice Calc and Google Sheets all open it without warnings.

How does it decide which cells are numbers?

Each cell is tested against a numeric pattern. Values like 42, -3.14 or 1.2e6 are written as true number cells so Excel treats them as numbers; everything else, including dates and IDs with leading zeros, is written as text to avoid silent reformatting.

Which delimiters are supported?

Comma, semicolon, tab and pipe. The CSV parser follows RFC 4180, so quoted fields, escaped double quotes and values containing embedded commas or newlines are handled correctly.

Is my data uploaded to a server?

No. Parsing and workbook generation happen entirely in your browser using JavaScript and the Web APIs. Your data never leaves your device, so you can safely convert confidential exports.

Why does my number show as text in Excel?

Values that are not strictly numeric — for example a phone number with a leading +, an ID like 00123, or a date — are kept as text on purpose, because converting them would change their meaning. Remove the non-numeric characters if you genuinely want a number cell.

CSV to XLSX Converter

This tool converts a CSV file into a real Excel .xlsx workbook in your browser. Unlike renaming a file to .xls, which only produces a fake spreadsheet Excel complains about, this builds the proper OOXML format so the result opens cleanly everywhere and treats numbers as numbers.

How it works

There are three stages, all running locally:

CSV parsing. The text is parsed according to RFC 4180. Quoted fields are unwrapped, doubled quotes ("") become a single literal quote, and commas or newlines inside quotes are kept as part of the field rather than splitting it.
Cell typing. Each value is checked against a numeric pattern. Genuine numbers become Excel number cells; all other values become inline string cells, which preserves IDs, dates and anything with leading zeros exactly as written.
Workbook assembly. The tool writes the required OOXML parts — [Content_Types].xml, the relationship files, workbook.xml, a small styles.xml for the optional bold header, and the worksheet itself — then packs them into a ZIP using the STORE method with a correctly computed CRC-32 on every entry. That ZIP, with an .xlsx extension, is a valid Excel workbook.

No spreadsheet library is loaded and nothing is sent anywhere; the entire converter is a few hundred lines of JavaScript running on your machine.

Tips and example

Given a CSV whose first line is name,age,city, leave Bold first row ticked to get a styled header, and the age column will arrive as real numbers you can sum or sort. If your export uses semicolons (common in European locales) switch the delimiter to semicolon so the columns split correctly.

For the reverse direction — pulling data back out of a spreadsheet — use the companion XLSX to CSV converter.