What happens if the two CSVs have different columns?

In the default header-union mode, the output header is the union of both files' columns. Any row from a file that's missing one of those columns gets an empty cell there instead of misaligned data.

What is positional stacking mode for?

Positional mode simply appends File B's rows after File A's rows without touching column order, which is faster and simpler when you already know both files share identical, same-ordered headers.

Does merging remove duplicate rows?

No, this tool only concatenates rows; it doesn't deduplicate. Run the output through a dedicated deduplication tool afterward if you need unique rows.

Which column order does the merged header use?

File A's columns come first in their original order, followed by any columns from File B that didn't already exist in File A.

CSV File Merger - Combine Two CSVs Online

Overview

Introduction

Combining two CSV exports by hand, in a spreadsheet, means manually copying rows and hoping the columns line up. This tool merges two CSV texts directly, aligning columns by header name so mismatched column sets don't corrupt your data.

It's built for the common case of appending a second export (a new batch of records, a different month's data) onto an existing dataset.

What Is CSV File Merger?

A two-input CSV merger that concatenates the data rows of File A and File B into one CSV, using either header-aligned union (the default) or simple positional stacking.

Header-aligned union computes the union of both files' column names and remaps every row into that shared column order before appending it, so a column present in only one file doesn't shift the rest of that row's values.

How CSV File Merger Works

Both inputs are parsed independently with the same RFC 4180-aware CSV parser used across this category. Their header rows are compared: header-union mode builds one combined header list, then rebuilds every data row (from both files) against that list, using an empty string for any column the row's source file didn't have.

Positional mode skips all of that column-matching and instead just requires both files to have the same column count, then stacks File B's rows directly beneath File A's.

When To Use CSV File Merger

Use it when you've exported the same kind of data twice (e.g. two months of a report) and want one combined CSV.

Header-union mode is the safer default whenever you're not 100% sure both files' columns are in the exact same order.

Often used alongside CSV Comparator, CSV Differ and CSV Cutter.

Features

Advantages

Header-union mode tolerates files with reordered, missing, or extra columns without corrupting rows.
Positional mode is a simple, predictable pure-append when you know both files match exactly.
Runs entirely client-side, so neither file ever leaves your browser.

Limitations

Column matching in header-union mode is by exact header text, so a typo or trailing-space difference between the two files' headers is treated as two distinct columns.
It does not deduplicate rows that appear in both files.

Header-union vs. positional merge modes

Header-union vs. positional merge modes
Mode	Column requirement	Behavior
Header-union (default)	None: any overlap works	Remaps every row to a combined column set, filling gaps with empty cells
Positional	Same column count required	Appends File B's rows directly beneath File A's, unchanged

Examples

Merging two files with different columns

Input

File A:
id,name
1,Ada

File B:
id,email
2,[email protected]

Output

id,name,email
1,Ada,
2,,[email protected]

The merged header is the union of both files' columns; each row fills in an empty cell for whichever column its source file didn't have.

Best Practices & Notes

Best Practices

Use header-union mode unless you've already confirmed both files' headers match exactly.
Check the output's row count against the sum of both inputs' row counts as a quick sanity check.
Run a deduplication pass afterward if the two source files might overlap.

Developer Notes

Header-union mode builds a Map from each source row's own header to that row's values before remapping, rather than assuming positional alignment, which is what lets it merge files whose columns are reordered relative to each other.

CSV File Merger Use Cases

Combining two monthly exports of the same report into one file
Appending a newly exported batch of records onto an existing dataset
Merging CSVs from two systems that use mostly overlapping but not identical column sets

Common Mistakes

Assuming positional mode will align columns by name: it doesn't, it only stacks rows, so mismatched column order silently scrambles data.
Not noticing that a header typo between the two files (e.g. "Email" vs "email") creates two separate output columns instead of one merged one.

Tips

If you expect duplicate rows, pair this with a deduplication tool afterward.
Preview both inputs' header rows side by side before merging to catch naming mismatches early.

CSV File Merger