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Revealing how Excel workbooks are actually stored as zipped XML files

Did you know that Excel workbooks are really just a collection of XML files, all zipped together? We didn’t either, so we thought we'd tell the world.

Posted by Andy Brown on 16 May 2014

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The true nature of Excel workbooks - zipped files

Trivia question: what format do Excel workbooks use? If your answer is a proprietary Microsoft format, you couldn't be further from the truth.

Let's create a simple Excel workbook and dissect it:

Owly workbook

A simple workbook containing 3 sheets

Here's what this looks like in Windows Explorer:

Workbook in Explorer

The workbook above.

So far, so what. But let's now change the file extension from .xlsx to .zip (ignoring any complaints from Microsoft):

The zipped workbook

You can now see that the workbook is actually a collection of zipped files.

When you double-click on the file now, you can see its true contents:

The workbook contents

The true contents of the workbook: the worksheets folder contains one XML file for each worksheet.

Here's a sample of the first worksheet:

A worksheet as XML

The XML underlying each worksheet is completely transparent.

As Michael Caine apparently didn't say: "Not a lot of people know that!".

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