twineconvert

free · in-browser · no upload

CSV to XLSX
Converter

Drop your CSV file. We'll convert it to XLSX right here in your browser, your file never leaves your device.

or drop your file

Select your file here to get started

or drop your file here.

Accepts .csv

nothing uploaded no file size cap no signup

How it works

Three steps. No upload, no signup.

  1. 1

    Drop your file

    Click the dropzone above or drag a CSV from your desktop. Files of any size, there's no upload, so there's no upload limit.

  2. 2

    Convert in your browser

    The conversion runs entirely in this tab using JavaScript and WebAssembly. Your file never touches our servers, we don't have any.

  3. 3

    Download

    Get your XLSX the moment the conversion finishes. Convert another, or close the tab.

Files stay on your device

Your file is never uploaded. The entire conversion runs in your browser using WebAssembly. We can't see what you convert because we have no server to see it.

No file size limit

Server converters cap free users at 1-2 GB and gate larger files behind a paid plan. Since nothing uploads, our limit is whatever your browser can handle.

Free, no signup, no ads on conversions

No account required. No watermark on the output. No queue. Drop a file, get a converted file.

Why convert CSV → XLSX

What this conversion is actually for

CSV opens in Excel but loses all the things a real workbook needs: number/date formatting, multiple sheets, frozen headers, the ability to add formulas. Converting to XLSX gives the recipient a proper editable workbook instead of a raw text file Excel sometimes mangles (leading zeros, big numbers, dates).

A real example

You exported a report as CSV but the client opens it in Excel and the order IDs `00451` show as `451`, and the phone numbers turned into scientific notation. Convert to XLSX where text columns stay text, the client sees the data correctly without manually importing.

Troubleshooting

Numbers I wanted as text (IDs, ZIPs) still look numeric.

We type-detect conservatively. Values with leading zeros or that look like identifiers are kept as text; clearly numeric columns become numbers. If a specific column must be forced to text, prefix the CSV values with a single quote or format the column as Text in Excel after opening.

Formats involved

About CSV and XLSX

CSV, Comma-Separated Values

CSV is plain text, one row per line, fields separated by commas. The simplest possible tabular data format, which is exactly why it remains the most portable: every spreadsheet, database, programming language, and analytics tool reads CSV. Tradeoffs: no formulas, no formatting, no multiple sheets, and various edge cases around quoting fields that contain commas or newlines.

How to open

Excel, Google Sheets, Apple Numbers, LibreOffice Calc, any text editor, every database import wizard, every programming language with one line of code.

Full CSV format guide →
XLSX, Microsoft Excel Spreadsheet

XLSX is the spreadsheet format Excel has used since 2007, like DOCX, it's a zip containing XML for cells, formulas, formatting, and embedded objects. Replaces the older binary .xls format. Read and written by Excel, Google Sheets, Apple Numbers, and LibreOffice Calc, with high fidelity for standard cell data and reasonable fidelity for complex formulas and pivot tables.

How to open

Microsoft Excel, Google Sheets, Apple Numbers, LibreOffice Calc. CSV is a more portable format if you only need the raw cell values.

Full XLSX format guide →

Related tools

Convert other files to XLSX

Convert your CSV to other formats

FAQ

Common questions

Is this CSV → XLSX converter really free?

Yes. No signup, no watermark, no daily file count limit. Every conversion runs in your browser, your file never touches our servers because there are no servers.

Where does my file go when I convert it?

Nowhere. The conversion runs entirely in your browser using JavaScript and WebAssembly. Your file is never uploaded to our servers. We don't have any servers handling files, there's nothing for us to log, store, or accidentally leak.

What's the maximum file size?

Whatever your browser can hold in memory. Practically, this means a few hundred MB on most computers, significantly larger than the 1-2 GB caps that server-upload converters charge for. Very large files (multi-GB) may require closing other browser tabs first.

Why convert CSV to XLSX?

Universal tabular data interchange. Spreadsheets with formulas, formatting, multiple sheets. The most common reason to convert is compatibility, XLSX works in places where CSV doesn't, or vice versa.

How do I open a CSV file in the first place?

Excel, Google Sheets, Apple Numbers, LibreOffice Calc, any text editor, every database import wizard, every programming language with one line of code.

Does this work offline?

Once the page is loaded, the conversion itself runs entirely offline. The first time you use a tool, your browser downloads the conversion library (a one-time cache). If you reload while offline, the page won't load, but you can install the site as a Progressive Web App for full offline use.

Can I convert multiple files at once?

Single file at a time for now. Batch conversion is on the roadmap, for now, drop one file, download the result, then convert the next.