RunToolRunCSV Merger
Merge multiple CSV files.
How to Merge CSV Files
Select multiple CSV files
Click the upload area or drop all the .csv files you want to combine.
Merging runs locally
Rows from every file are combined under one header row in your browser.
Download the merged CSV
Get a single file ready for Excel, databases or reporting tools.
Merging assumes the files agree
Stacking several CSV files into one works cleanly when they share the same columns in the same order. That is the case for files exported from the same system in different periods — monthly reports, daily logs, split exports — which is what most merging is.
When the columns differ, something has to give: either the union of all columns with blanks where a file lacked one, or only the columns every file has. Neither is wrong, and picking silently is how data goes missing.
Header rows
Every file has a header, and the merged file needs exactly one. Keeping them all leaves header rows scattered through the data, which breaks any calculation and is surprisingly easy to miss in a file of thousands of rows.
The reverse mistake is dropping the first data row of each file by treating it as a header when the file has none. Checking the row count — total rows in, minus the number of files, plus one — catches both errors immediately.
Column order and quiet misalignment
Two files can have the same columns in a different order. Merging by position rather than by name then puts telephone numbers under 'email' and nobody notices until someone tries to send something.
This is the most dangerous failure in merging, because the result looks perfectly valid. Matching on column names rather than position is the fix, and spot-checking a few rows from each source file afterwards is how you confirm it worked.
Separators, encodings and line endings
Files from different systems can use different separators and different encodings. A semicolon-separated file merged with a comma-separated one produces rows that parse into completely different shapes, and a legacy-encoded file merged with a UTF-8 one produces mangled characters in part of the output.
Line endings differ too, and a merged file with mixed endings confuses some tools. Normalising to one convention on the way out avoids a class of problems that appear only in whatever reads the file next.
Duplicates across files
Overlapping exports are common — two monthly files that both contain the last day of the month, or an export re-run after a fix. Merging preserves both copies, and the duplicates are only visible if you look for them.
Deduplication is a separate step and needs a decision about what makes two rows the same. Identical text is easy; the same order appearing twice with a different timestamp is a judgement about the data rather than about the file.
Verifying the merge
Add up the data rows of the input files and compare against the output. Then check the first and last row of each source file appears in the result. Those two checks catch truncation, header confusion and misalignment together.
Merging runs in your browser, so sales exports, customer lists and financial records are never uploaded.