Base64 to CSV: Decode and Save CSV Files

How to decode Base64 to CSV

1. Paste the Base64 into Input · Base64. A short sample is loaded when the page opens.
2. Check the rows in Output · text. If the delimiter, quotes or accented letters look wrong, you know before opening the file anywhere else.
3. If characters show as �, set Charset to the encoding of the export, often Windows-1252 for files from older Windows software.
4. Press Download. When the decoded data looks like comma-separated rows, the status line reports CSV and the file is saved as decoded.csv. Short files and files separated by semicolons or tabs are reported as Text and saved as decoded.txt; rename those to .csv.

Download writes the decoded bytes unchanged, whatever charset you picked for the preview.

Example

The sample aWQsbmFtZSxlbWFpbAoxLEFkYSxhZGFAZXhhbXBsZS5jb20KMixMaW51cyxsaW51c0BleGFtcGxlLmNvbQo= decodes to:

id,name,email
1,Ada,ada@example.com
2,Linus,linus@example.com

Notice the start: aWQs is id,. CSV exports do not share a common prefix the way file formats do, but a few patterns help:

Where Base64 CSV files come from

Reporting and analytics APIs often return an export as a Base64 field inside a JSON response, because JSON cannot hold raw file bytes. Webhooks and message queues do the same for attachments, and automated emails carry CSV attachments as MIME Base64 parts; the email decoder extracts those from a raw message. Git hosting APIs return file contents as Base64 with line breaks, which this decoder skips.

If the JSON response holds several encoded fields, decode Base64 inside JSON finds them all at once.

Opening the decoded file in Excel

CSV has no header that declares its charset, so spreadsheet programs guess:

• With a UTF-8 BOM (Base64 starting with 77u/), Excel reads accents and non-Latin scripts correctly when you double-click the file. 77u/aWQ7bmFtZTtjaXR5CjE7Wm/DqztLw7Zsbgo= is such a file, with Zoë and Köln intact.
• Without a BOM, Excel on Windows uses the system’s legacy code page, and UTF-8 text turns into Zoë. Use Data > From Text/CSV and choose UTF-8 instead of double-clicking.
• Semicolons instead of commas are normal in countries where the comma is the decimal separator. Excel opens such files correctly only when the system’s list separator is a semicolon; otherwise use the import dialog and set the delimiter.

Base64 to CSV in code

import base64, csv, io

s = "aWQsbmFtZSxlbWFpbAoxLEFkYSxhZGFAZXhhbXBsZS5jb20KMixMaW51cyxsaW51c0BleGFtcGxlLmNvbQo="
raw = base64.b64decode(s)

with open("export.csv", "wb") as f:
    f.write(raw)

rows = list(csv.reader(io.StringIO(raw.decode("utf-8-sig"))))
rows[1]   # ['1', 'Ada', 'ada@example.com']

utf-8-sig removes a BOM if there is one and is harmless otherwise. On the command line, base64 -d export.b64 > export.csv does the same as the Download button. See Base64 in Python for streaming large files.

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