How can it read .xlsx without Excel or a library?

An .xlsx file is a ZIP archive of XML parts. The tool reads the ZIP directly and inflates each part using the browser's built-in DecompressionStream, then parses the worksheet XML and the shared-string table. No spreadsheet library is loaded.

Which cell types are handled?

Shared strings, inline strings, plain string cells, numbers and booleans. Shared-string indices are resolved against the sharedStrings table so text appears correctly, and boolean cells are emitted as TRUE or FALSE.

Does it handle multiple sheets?

Yes. Every worksheet is parsed and you can switch between them with a dropdown. Each sheet exports to its own CSV, since CSV is a single-table format and cannot hold more than one sheet at once.

What about formulas and formatting?

CSV is plain text, so formatting, colours and merged-cell styling are dropped. Formula cells export their last calculated value, which Excel stores alongside the formula, so you get the result rather than the formula text.

Is my spreadsheet uploaded?

No. The workbook is read, decompressed and parsed entirely on your device with JavaScript and Web APIs. Your data never leaves the browser, so confidential spreadsheets stay private.

XLSX to CSV Converter

This tool converts an Excel .xlsx workbook into CSV without Excel, a server round-trip, or any external library. Pick a file, choose a sheet and delimiter, and get clean RFC-4180 CSV you can drop into a database import, a data pipeline, or a script.

How it works

An .xlsx file looks like a single document but is actually a ZIP archive containing several XML files. The converter takes advantage of that structure using only browser-native APIs:

Unzip. It walks the ZIP’s local file headers and decompresses each entry with the built-in DecompressionStream("deflate-raw"), the same DEFLATE algorithm the archive was written with.
Shared strings. Text in .xlsx is usually stored once in xl/sharedStrings.xml and referenced by index from cells, which keeps files small. The tool parses that table first.
Worksheets. Each xl/worksheets/sheetN.xml is parsed cell by cell. The cell’s t attribute tells the parser whether it is a shared-string index (s), an inline string (inlineStr), a plain string (str), a boolean (b) or a number, and the value is resolved accordingly. Cell references like C5 are decoded so blank cells are padded and columns stay aligned.
CSV output. Rows are joined with your chosen delimiter, and any value containing the delimiter, a quote, or a newline is quoted and its quotes doubled, per RFC 4180.

Tips and notes

Formulas export as their last calculated value, because Excel caches that value next to the formula. If a workbook was never opened in Excel after the formula was written, that cached value may be missing.
Formatting is not data. Number formats, colours and merged cells do not survive the trip to CSV; only the underlying values do. A cell displayed as £1,234.50 exports as 1234.5.
Use the semicolon delimiter if your downstream tool expects the European CSV convention, and tab to produce a TSV that pastes cleanly into other spreadsheets.

To go the other way and build a workbook from CSV, use the companion CSV to XLSX converter.