Explicit headers · Local processing
Merge CSV Files
Combine two or more CSV files without uploading their contents. Require an exact header match or deliberately create an ordered union.
Merge CSV files locally
Choose two or more files, up to 50 MB combined. Nothing is uploaded.
Predictable by default
No silent column shifts
Strict mode prevents a value from sliding under the wrong column just because one file changed header order. Union mode is available when files intentionally contain different columns.
Union mode requires unique, nonblank headers in every file. That makes the mapping explicit instead of guessing which duplicate or blank column a value belongs to.
Plain answers
CSV merge questions
What is strict header matching?+
Every file must have exactly the same source headers in the same order. A mismatch names the file that needs attention and blocks the download.
How does header union work?+
Union mode keeps headers in first-seen order, maps cells by exact header text, and leaves a cell empty when a file does not contain that header.
Are merged files uploaded?+
No. Selected files are parsed, combined, and downloaded in this browser tab. Only ordinary page and code requests reach the hosting service.