CSV to XLSX Convert
Turn CSV or TSV files into an Excel workbook, with one sheet per file and numbers stored as numbers.
CSV to XLSX is a free online tool that turn CSV or TSV files into an Excel workbook, with one sheet per file and numbers stored as numbers. It runs entirely in your web browser using JSZip, so files are processed on your own device and never uploaded to a server. There is no sign-up, no file size limit imposed by the site, no watermark and no paid tier.
- Price
- Free: no account, no quota, no watermark
- Category
- Convert
- Where it runs
- In your browser, on your device
- Files uploaded
- None
- Technology
- JSZip
- Settings
- 6
- Works offline
- Yes, after the first visit
About CSV to XLSX
An .xlsx file is a ZIP archive of XML parts, so this tool writes those parts itself: a workbook index, a style table with a bold header style, and one worksheet per CSV file. Each field is parsed following RFC 4180, then written as a number, a boolean or a text cell. Values that only look numeric, such as IDs with leading zeros or anything longer than 15 digits, stay text so Excel cannot mangle them. Drop several files and each becomes its own sheet, named after the file. Nothing is uploaded and no spreadsheet library is downloaded.
How to use it
- Drop one or more CSV or TSV files on the page, or paste data into the box. Each file becomes a sheet.
- Check the delimiter and the workbook file name, and untick any of the header, freeze, type or width options you do not want.
- Press "Convert to XLSX" and download the workbook, or open it straight in the XLSX Viewer to check it.
The 6 settings
| Setting | What it does | Default |
|---|---|---|
| Delimiter | Choose from 5: Detect automatically, Comma, Semicolon, Tab, Pipe. | Detect automatically |
| Workbook file name | Free text. The .xlsx extension is added if missing. | data.xlsx |
| First row is a header (bold) | On or off. Makes the first row bold. | On |
| Freeze the header row | On or off. Keeps the header visible when scrolling in Excel. | On |
| Store numbers and booleans as such | On or off. When off, every cell is text. | On |
| Fit column widths to content | On or off. Sizes each column to its longest value, up to 60 characters. | On |
Under the hood
| Runs on | JSZip: packs the workbook; plain JavaScript parses the CSV and writes the sheet XML |
| Controls | Delimiter, Workbook file name, First row is a header (bold), Freeze the header row, Store numbers and booleans as such, Fit column widths to content |
Questions
Why did my ID column stay as text?
On purpose. A value with a leading zero, more than 15 digits, or a plus sign is not written as a number, because Excel would drop the zeros or round the digits. Phone numbers, product codes and account numbers are exactly the cases that go wrong in Excel's own CSV import, so they are protected here.
Does it write formulas or dates?
No. Cells are numbers, booleans or text. A date string such as 2026-09-05 is stored as text, which Excel displays as typed. If you need real Excel dates, convert the column inside Excel with Text to Columns or DATEVALUE.
Can I put several CSV files into one workbook?
Yes. Drop them all at once, or one after another. Each file becomes a sheet named after the file (trimmed to the 31 characters Excel allows), in the order you added them.
Will Excel complain that the file is corrupt?
It should not. The workbook is written with the same parts Excel itself produces: content types, relationships, a style sheet and worksheets with inline strings. It opens in Excel, LibreOffice, Numbers and Google Sheets. If a file does fail, the cause is nearly always a CSV that was not really UTF-8; re-save it as UTF-8 and try again.
Can it produce the old .xls format?
No. The pre-2007 .xls format is binary and much more involved. Every current spreadsheet app opens .xlsx, and Excel can Save As .xls if something still needs it.
How large a file can it convert?
There is no site limit. The whole file is parsed in memory and the workbook is compressed on the fly, so a few hundred thousand rows convert in seconds on a laptop.
Can I use it full screen, or keep a copy offline?
Yes to both. "Full screen" above the tool fills the whole screen (Esc leaves it). "Save as HTML" writes this one tool to a single self-contained file that opens from your own disk without a connection. For everything at once there is a desktop app for Windows, macOS and Linux with all 312 tools inside it.