SpreadsheetLosslessRuns in your browser

Merge CSV

Stack several CSVs into one, aligning columns by header name. The conversion is lossless — every pixel, sample or byte in the source is preserved.

Not uploaded to any serverConverted on this deviceGone when you close the tabHow this works

Drop your files here

CSV and TSV · up to 100 MB each

or paste files from your clipboard

Ready when you are

Stack several CSVs into one, aligning columns by header name. Everything happens in this tab; nothing is sent anywhere.

⌘K to search converters

How to use it

1
Add your files
Merge CSV works with CSV and TSV. Drop them in, browse for them, or paste from your clipboard.
2
Put them in the right order
The files are processed top to bottom. Drag them to rearrange before you start.
3
Run it and download
Press the button and save the result. Everything happens on your device, and nothing is uploaded.

Privacy, in plain words

Is it safe? Your spreadsheet is not uploaded anywhere

What “uploaded” normally means. On most online converters, pressing the button copies your file across the internet onto a computer owned by that website. That copy is what the word “upload” means, and it is the part nobody explains.

What happens instead. This site does not do it. There is no server here that receives files, so your spreadsheet is read, converted and handed back by the browser you are using right now. It never travels — so there is no copy to store, read, leak or hand over.

Check it yourself in ten seconds

Turn off your Wi-Fi after this page has loaded, then convert a file anyway. It still works, because everything needed is already on your device.

The longer explanation of how this works, and the full detail of what is and is not stored on the privacy page.

Why this matters more than it sounds

A spreadsheet is rarely about one person. It is usually about hundreds of them, which is what turns an ordinary file into a reportable data breach. It matters more than it sounds, because of what spreadsheets usually are:

Supplier pricing

commercially confidential by contract, usually

Donor and volunteer databases

people who trusted an organisation, not a converter

Payroll

every colleague's salary, in one file

Customer and member lists

names, emails, phone numbers, addresses

A file that is never sent cannot be kept, leaked, subpoenaed or used to train anything. That is a property of how this works, not a promise about how carefully it is run.

How Merge CSV works

Stack several CSVs into one, aligning columns by header name. Everything happens inside this page: the file is read from your device into memory, processed there, and handed back as a download. It is never sent anywhere, and closing the tab discards it.

What it accepts: CSV, TSV. Files are identified by their contents rather than their name, so a mislabelled file is still handled correctly.
What you get back: A CSV file, downloaded straight to your device.
What happens to the data: Nothing is re-encoded. The content that comes out is identical to the content that went in, byte for byte, and running it twice changes nothing.
Size limit: Up to 100 MB per file. Because the work happens on your own machine, available memory is the real constraint — a laptop handles far more than a phone.

The settings, and what each one changes

Merge CSV exposes 7 settings. The defaults are chosen to suit the common case, so changing nothing is a reasonable way to start.

Delimiter: Choose between Detect automatically, Comma, Semicolon, Tab and others.
First row is a header: On or off. Off is the default for merge csv unless stated otherwise.
Sheets: Choose between First sheet only, Every sheet.
Remove duplicate rows: On or off. Off is the default for merge csv unless stated otherwise.
Trim surrounding whitespace: On or off. Off is the default for merge csv unless stated otherwise.
Remove empty rows: On or off. Off is the default for merge csv unless stated otherwise.
Compare: Choose between Whole row, First column only.

Worth knowing before you start

Columns are matched by header, so files with different column orders still line up. Missing columns are filled with blanks rather than silently shifting your data.

Measured, not estimated

What this tool actually does to a file

Merging two copies of the same sheet into one file.

The result came out 2.0× smaller than the file that went in.

Each row is one document, chosen because this tool behaves differently depending on what is inside the file.
Sample	In	Out	Change
A plain tableTen rows of flat records — the shape almost every spreadsheet export actually has.	760 B	380 B	−50%
Commas and quotation marksText fields containing the delimiter and the quote character — where an escaping bug would show.	1.7 KB	875 B	−50%

Produced by this site's own converter. The same code that runs in your browser, at the same default settings, so these are the bytes you would download.

Samples are generated from a fixed seed rather than taken from real photographs or documents, so the corpus is reproducible and carries no licensing question — and so the numbers predict the shape of your result rather than its exact size. Measured 2026-10-01. The method, and the command that regenerates every figure on this page, are on the how it works page.

About these formats

Accepts

Comma-Separated Values

Flat tabular text — one table, no formatting.

Extensions: .csv
MIME type: text/csv
Best for: Data imports and exports · Databases · Scripts
Compatibility: Every spreadsheet, database and programming language.

Accepts

Tab-Separated Values

Like CSV but tab-delimited — safer for text with commas.

Extensions: .tsv, .tab
MIME type: text/tab-separated-values
Best for: Data pipelines · Text containing commas
Compatibility: Wide, especially in data tooling.

Columns are matched by header, so files with different column orders still line up. Missing columns are filled with blanks rather than silently shifting your data.

Good to know

Merge CSV — questions

How does Merge CSV work?

Stack several CSVs into one, aligning columns by header name. Add your files above, adjust anything you want to change, and press the button. The result downloads straight to your device.

Is there anything I should watch out for?

Columns are matched by header, so files with different column orders still line up. Missing columns are filled with blanks rather than silently shifting your data.

Is Merge CSV safe — are my files uploaded to a server?

No, nothing is uploaded. On most online tools, pressing the button copies your file across the internet onto a computer owned by that website, where it is processed and then sent back. That copy is what "upload" means. This tool does not do it: your file is read and processed by the browser you are using right now, stays on this device the whole time, and is discarded when you close the tab. Turn off your Wi-Fi after the page loads and it still works, which is the quickest way to confirm it.

Can I use this on confidential or personal documents?

Yes — that is the case it was built for. Because the file never leaves your device, a confidential document is no more exposed than it is sitting in your own folder: there is no transfer, no copy on anybody else's hardware, no retention period and no staff with access. Nothing about the file is logged either — not its name, not its size, not its contents.