How Browser-Based CSV Merging Works
Understanding how our tool processes your CSV files entirely in your browser - no uploads, no servers, complete privacy.
100% Private
Files never leave your computer. All processing happens locally in your browser.
Fast Processing
No upload wait time. Processing starts immediately using your computer's power.
No Limits
Only limited by your browser's memory. Handle files with millions of rows.
The Process, Step by Step
File Selection
You select CSV files from your computer using the file picker or drag-and-drop. Files are read into browser memory using the FileReader API.
Technical detail: The browser creates a reference to files on disk. They're not uploaded anywhere - they're accessed locally in read-only mode.
CSV Parsing
Each file is parsed from raw text into structured data (rows and columns). JavaScript reads the file and splits it into:
- • Headers: Column names from the first row
- • Rows: Data rows as arrays
Handles edge cases: Quoted fields, commas in values, different line endings (Windows/Mac/Linux), escaped quotes.
Merging
Files are combined according to your selected merge type:
Vertical Merge (Stack Rows)
All rows from all files are stacked top to bottom. All columns from all files are included. Missing values are filled with empty cells.
Horizontal Merge (Join by Key)
Files are joined side-by-side based on a common key column (like VLOOKUP or SQL JOIN). You choose: normal join (keep all rows), match join (only matching rows), or filter (only non-matching).
Preview & Download
The merged data is converted back into CSV format (text with commas and newlines). You see a preview of the first 5 rows. When you click download, the browser saves it directly from memory.
No server involved: The download happens using the Blob API and URL.createObjectURL(). The file goes straight from browser memory to your disk.
Vertical vs Horizontal Merge
Vertical Merge
Stack rows from multiple files top to bottom.
Use when: Combining data from the same source (e.g., monthly exports, multiple locations, multi-part downloads).
Horizontal Merge
Join files side-by-side by matching a key column.
Use when: Enriching data from different sources (e.g., combining customer details with transaction data).
Privacy & Security
No Server Upload
Files are never uploaded to any server. They're read directly from your disk into browser memory and processed there.
No Data Collection
We don't see your files, file names, or any data. There's no analytics on file content. Only basic page view tracking (Google Analytics).
Works Offline
After the page loads, you can disconnect from the internet and it still works. All processing is local.
Limitations to Be Aware Of
Browser Memory
Very large files (multi-GB) can slow down or crash your browser tab if you don't have enough RAM. Close other tabs if you're merging huge datasets.
File Encoding
Works best with UTF-8 encoded CSV files. Other encodings might display special characters incorrectly.
Browser Compatibility
Requires a modern browser (Chrome, Firefox, Safari, Edge) with JavaScript enabled. Internet Explorer not supported.
Ready to Try It?
Merge your CSV files with complete privacy. Free, fast, no signup required.
Start Merging CSV Files