Withdata DataFileConverter merges CSV files into one JSON file locally — any number of files, mapped column by column, with the output options of your choice. 100% local, secure, and cross-platform.

Why Choose Withdata DataFileConverter as a CSV Merger?

🔒 100% Local & Secure

All merging runs on your own computer. Nothing is uploaded to any cloud server.

⚡ Fast Streaming Merge

Millions of rows within a minute. Large files are streamed with low memory usage.

📁 Any Number of Files

Add two files or a whole folder, then combine everything into a single JSON file.

🔄 Column Mapping

Files with a different column order or extra columns can be mapped to the output columns before merging.

🖥 Windows, macOS, Linux

Native builds for Windows, macOS and Linux, with a step-by-step GUI wizard and a command line interface.

⏱ CLI & Scheduled Tasks

Save a merge session and run it automatically from Windows Task Scheduler or crontab.

Full Feature Comparison

Feature	Supported details
Input	Any number of CSV files, added one by one or as a folder
Output format	JSON object array, plain JSON array, or JSON Lines (one object per line)
Merge mode	All input rows into one output file
Running mode	GUI wizard · command line · scheduled tasks
Column handling	Source columns are mapped to the output columns
Output options	Object array / plain array / JSON Lines, date format
Platform	Windows, macOS, Linux (native, 64-bit)

CSV to JSON — What Changes

Every CSV row becomes one JSON object and the whole set is written as an array. Header cells become the keys, and each value starts as a string unless the column has an explicit type.

All CSV Merge Options You Need

Merge several CSV files into one JSON file — step by step in the GUI wizard
Merge a whole folder of CSV files in one run
Map the columns of each input file to the output columns
Run the merge from the command line, and schedule it as a scheduled task
All CSV tools and all JSON tools
Guide: Merge multiple CSV files into one JSON

Same source format, other targets

This page merges CSV into another format. If you need a different output, these pages each do that one job:

All merge tools, by format

Merging into JSON — Notes

Nested objects are flattened into dotted column names when the files are read.
The delimiter and quote character can be chosen; quoted fields and embedded line breaks are parsed correctly.

How to Merge CSV Files (4 Steps)

Open Withdata DataFileConverter and choose Merge (combine multiple files into one).
Add the CSV files you want to merge — two files, a list, or a whole folder.
Check the preview and map the source columns to the output columns; then set the output options (Object array / plain array / JSON Lines, date format).
Click Convert — the single JSON file is written in seconds. Save the session if you want to run it again from the command line.