Merge CSV Files

Stacking puts the rows of every file one after another, which is what you want when the files are the same shape — twelve monthly exports, say. Joining matches rows across files using a key column such as an ID or an email, which is what you want when each file holds different columns about the same people.

You are told before anything is written, with the differing column names listed. Silently stacking files whose columns are in a different order is how data ends up in the wrong field, so the mismatch is surfaced rather than guessed around.

Yes. There is an option to add a column recording the source filename, which is the thing you always wish you had added once the merged file is a hundred thousand rows long and one of them looks wrong.

For the common case, yes: joining on a key column does what a VLOOKUP across two sheets does, without the formula. What it does not do is approximate matching or anything conditional. It reports unmatched rows explicitly, which a VLOOKUP quietly turns into #N/A.

Yes. Each file’s separator is detected on its own, so a comma-separated export and a semicolon-separated one can be merged together, and you choose the separator for the result.

Yes. There is no account, no daily cap, no file-size tier and no watermark, because there is no server cost to recover. The site is funded by clearly disclosed partner recommendations on the Partners page, never by metering the tools.

No. Everything is processed by JavaScript running in your browser, on your device. You can verify it: open your browser's developer tools, switch to the Network tab, and use the tool. You will see no request carrying your data — because there is nowhere for it to go.