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TheSpreadFix

Merge CSV & Excel Files

Combine two files in your browser. Append mode stacks rows vertically — ideal for monthly exports or regional data. Join mode works like VLOOKUP: it matches rows by a key column and adds columns from the second file. Inner, left, right and full joins are supported.

File A (base / first)

Drop a CSV or Excel file

File B (second / to add)

Drop a CSV or Excel file

Both files stay on your device. They're merged in your browser — nothing is uploaded.

How it works

  1. Step 1

    Drop both files

    Choose the base file (A) and the file to combine (B). CSV and XLSX are both supported.

  2. Step 2

    Pick a mode

    Append stacks rows one after the other. Join matches rows by a key column and combines columns.

  3. Step 3

    Download the result

    Preview the merged data, then download as CSV or Excel.

What it checks and fixes

Append with column alignment

Columns from both files are matched by name and known aliases. Unmatched columns become new columns. A Source file column tells you where each row came from.

Four join types

Inner (only matches), Left (all from A + matches from B), Right (all from B + matches from A) and Full (everything from both files).

Duplicate column handling

When both files have a column with the same name (other than the key), the second is renamed automatically to avoid confusion.

Multi-match reporting

If a key matches multiple rows in the second file, every combination is kept and the count is reported so you know.

When to merge CSV files

Data often comes in pieces. Monthly sales exports, regional branch data, system exports split by date — appending them into one file is the first step before analysis.

Joining is useful when you need to enrich one file with columns from another: adding customer details to an order export, attaching prices to a product list, or filling in contact info from a CRM export.

Append vs. Join — which one do I need?

Append (vertical) is for files with the same columns but different rows. Think of stacking January's orders on top of February's.

Join (horizontal) is for files that share a key column but have different information. Think of matching an order list with a customer list by email.

If you're unsure: if the files have mostly the same column names, append is probably what you want. If they share one key column but have different data, try a join.

Frequently asked questions

Can I merge more than two files?

The tool accepts two files at a time. For three or more, merge the first two, download the result, then merge it with the third.

What if the column names don't match exactly?

The tool detects common aliases (e.g. 'email' and 'E-mail Address'). You can also manually choose which column maps where.

Is my data uploaded?

No. Everything runs in your browser. Your files stay on your device.

What's a full join?

A full join keeps every row from both files. Matched rows are combined; unmatched rows appear with blank values for the missing file's columns.