Base64 to Excel (XLSX)

How to decode Base64 to an Excel file

1. Paste the Base64 into Base64 or data URI. A prefix such as data:application/vnd.openxmlformats-officedocument.spreadsheetml.sheet;base64, can stay. For a long export saved as text, use Open .txt file.
2. Above the table, a line names the sheet: “First sheet: “Invoices” (3 in the workbook)”. The table has column letters across the top and the sheet’s own row numbers down the side, so row 17 is row 17 in Excel even if rows before it are empty.
3. Below the table, the entry list shows every part of the workbook with View and Save.
4. Press Download file to save the .xlsx.

Reading the sheet does not need Excel or any add-in. The page unpacks the workbook and parses its XML in your browser, and its Content Security Policy allows no network requests except Cloudflare’s page-view count, so the data stays on your device.

How the preview shows cell values

The table shows what is stored in each cell, without Excel’s number formats. That is usually what you want when checking an export, but a few kinds of cells look different from Excel:

To turn a serial number into a date, add that many days to 30 December 1899: datetime(1899, 12, 30) + timedelta(days=45672) gives 15 January 2025. Workbooks that use the 1904 date system, a setting from old Mac versions of Excel, count from 1 January 1904 instead.

Formula results appear only if the program that wrote the file stored them. Excel and LibreOffice do; some libraries write formulas without results, and those cells show up empty.

The preview has limits: it reads the first sheet in workbook order (the leftmost tab, even if it is hidden or another tab was active), up to 200 rows and 30 columns (A to AD). When there is more, the note “Showing up to 200 rows × 30 columns.” appears. Charts, images, comments and formatting are not shown.

XLSX, XLS, CSV or something else

Two ZIP-based variants need attention. A workbook with macros contains xl/vbaProject.bin; it previews normally, but Excel refuses to open it under the .xlsx extension, so type xlsm before downloading. A binary workbook (.xlsb) keeps its sheets in .bin files instead of XML: the tool still calls it XLSX and lists its entries, but shows no table, and it has to be saved as xlsb.

A CSV sent instead of a workbook triggers “This looks like CSV, not XLSX.” with a Download as .csv button. Excel opens either.

Where Base64 spreadsheets come from

Reporting and accounting APIs return generated exports as a Base64 field in JSON. Automation and low-code platforms often pass files between steps as Base64 strings, and email carries spreadsheet attachments as Base64 MIME parts. For the opposite direction, from a workbook to a string, use Excel to Base64.

Read a Base64 workbook in Python

With the openpyxl package, the workbook can be read straight from the decoded bytes:

import base64, io
from openpyxl import load_workbook

with open("export.b64") as f:
    raw = base64.b64decode(f.read())

wb = load_workbook(io.BytesIO(raw), data_only=True)
ws = wb.worksheets[0]
for row in ws.iter_rows(values_only=True):
    print(row)

data_only=True returns the saved formula results instead of the formulas, the same values the preview shows. Unlike the preview, openpyxl applies the number formats to dates, so the cell above comes back as datetime(2025, 1, 15, 0, 0). More decoding options are on Base64 in Python.

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