Muffin's Data Converter

Convert CSV, JSON, JSONL, and Excel files instantly — entirely in your browser. Nothing is uploaded anywhere.

// conversion mode:

Input

📂

Drop file or click to browse

CSV · JSON · JSONL · XLSX

Output

Output appears here after conversion…

// quick conversions — click to set

⇄ CSV → JSON

⇄ JSON → CSV

⇄ CSV → JSONL

⇄ JSONL → CSV

⇄ Excel → CSV

⇄ CSV → Excel

⇄ JSON → JSONL

⇄ JSONL → JSON

What is Muffin's Data Converter?

Muffin's Data Converter is a free, browser-based tool that lets you convert data files between the most common formats used in data science, research, and software development — CSV, JSON, JSONL, and Excel (.xlsx). Unlike most online converters, your file never leaves your device. Everything is processed locally in your browser using JavaScript, so there is no upload, no server, and no risk of your data being stored or shared.

Why do you need a data format converter?

Different tools, frameworks, and pipelines expect data in different formats. A machine learning engineer might receive experiment results as an Excel spreadsheet but need them as JSON to load into a Python script. A web developer might get API data as JSON but need it as CSV for a spreadsheet report. A researcher might have JSONL logs from a training run but need them as CSV to open in Excel or Google Sheets.

Instead of writing a Python script every time — or uploading sensitive data to a random website — Muffin does the conversion instantly in your browser. No code, no installs, no privacy risk.

Supported conversions

CSV → JSON

Convert spreadsheet-style data into structured JSON objects. Each row becomes an object with column names as keys.

JSON → CSV

Flatten a JSON array into a CSV table. Perfect for turning API responses into spreadsheet-ready data.

Excel → CSV / JSON

Read .xlsx files directly and convert to any format. No need to open Excel and export manually.

CSV → JSONL

Convert to newline-delimited JSON — the standard format for large dataset streaming, fine-tuning LLMs, and log files.

JSONL → CSV

Turn newline-delimited JSON logs or dataset files into a CSV you can open in Excel or Google Sheets.

CSV → Excel

Package your CSV data into a proper .xlsx file ready to share with colleagues who use Excel or Google Sheets.

Select only the columns you need

Muffin's converter includes a column filter mode — useful when your file has many columns but you only need a few. Switch to "Select columns to keep", load your file, and checkboxes appear for every column. Uncheck the ones you don't need and convert only the data that matters. This is especially useful for research datasets with dozens of metrics where you only need a subset for a specific analysis or report.

100% private — your data never leaves your browser

Most online converters upload your file to a server to process it. That means your data — which could contain sensitive research results, proprietary datasets, or personal information — is transmitted to a third party. Muffin processes everything locally using the browser's built-in JavaScript engine. The file is read into memory, converted, and downloaded — all without a single byte leaving your machine. You can even disconnect from the internet after the page loads and it will still work.