How to merge CSV files online
Add two or more CSV files, choose how to combine them, and merge. If every file has the same columns and delimiter, the rows are copied byte for byte and the header is written once, which is fast even for very large files. The files are streamed one after another, so the total size is limited by your disk, not by memory.
If the columns differ, choose Union: the merged file gets every column that appears in any file, in the order they first appear, and cells for columns a file does not have are left blank. Strict refuses to merge files whose headers differ, which is useful when a different header means something has gone wrong.
Merge CSV files with different columns
- Columns are matched by header name, not by position, so a file with the same columns in a different order still lines up.
- A column that exists in only one file appears in the merged file, blank for the rows from the others.
- Rows with more fields than their header have the extra values kept at the end of the row, and the count is reported.
- Files using different delimiters are combined into one file that uses the delimiter of the first.
Messy files, handled
- Quoted fields that contain commas, doubled quotes and line breaks are read correctly, so a note with a newline in it does not split into two rows.
- Comma, semicolon, tab and pipe delimiters are detected from the start of the file, including the semicolons European exports use.
- A byte-order mark, and Windows (CRLF) or Unix (LF) line endings, are handled.
- Rows with more or fewer fields than the header are counted and shown, not silently dropped.
- Files with no header row are supported with one checkbox.
- UTF-8 files are supported. A UTF-16 file needs saving as UTF-8 first, and the tool tells you so.
Tips for a clean merge
Check that every file has a header row and that the column names are spelled the same way; “Email” and “email address” are different columns to a merge. If you have hundreds of files, add them all at once — the order they are listed is the order the rows appear.
How to use it
- Drop two or more CSV files onto the box.
- Choose Union or Strict.
- Click Merge and wait for the progress bar.
- Download the merged file.
Frequently asked questions
Can I merge CSV files without uploading them?
Yes. The files are read and combined in your browser; nothing leaves your device.
What if the files have different columns?
Use Union to combine them by column name, leaving blanks where a file lacks a column. Use Strict if you want the merge to stop when the headers do not match.
How many files can I merge?
As many as you like. They are streamed one at a time, so memory does not grow with the number or size of the files.
How large a CSV file can it handle?
The largest file it has been run on is 1 GB, opened and scrolled in a real browser. It streams the file instead of loading it, so memory grows with the number of rows (about 8 bytes each for the index), not with the size of the file. Very large row counts, roughly above a hundred million, may exhaust a browser tab’s memory.
Is my file uploaded anywhere?
No. The file is read in pieces by code running in your browser and is never sent to a server. The site records only an anonymous counter with the operation and rough size buckets, such as “100–500 MB” and “1M–10M rows” — never file names, column names or any data.