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There have been several questions about this topic already:

ISO file much smaller than extracted data

Why is the windows ISO file much smaller than the installed OS?

These questions ask why is the iso file much smaller than the data contained within it. My question is different. I want to know how much smaller is the iso file relative to the data? What is the order of magnitude of compression? Is the iso file relatively 25% smaller than the data, 75% smaller? If my data has a size of X GB, then how large can I expect the iso file to be?

The reason I'm asking this question is because I want to duplicate an OS and knowing it's size, I want to find out the relative size of the completed iso file, so that I can buy a usb drive in advance that is big enough to hold that iso file. I will be saving/burning the iso file directly onto the drive.

Galaxy
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    You cannot know in advance. It depends on what files are going into the iso, what their contents are, and what compression method is used. Remember that some files will compress more than others depending on their contents. – DavidPostill Jan 04 '19 at 21:27
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    It should be noted that an ISO file is just a container. It does not support compression on its own, so it will be the same size as the data you put on it. In the two examples you provided, the data was compressed for the Windows ISO, and deduplicated in the Visual Studio ISO. Both are merely optimization techniques that Microsoft employed to get the ISO size down. You don't get those features for free. – Wes Sayeed Jan 04 '19 at 22:55

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The amount a compressed file is reduced in size is completely dependent on the type of file, content and method of compression.

So, there is no arbitrary formula for answering your question.

The only reasonable advice that can be given is that you should purchase a flash drive that is at least as big as the entire content of uncompressed data you intend to save on it. Then you can be sure you’ll have enough space.

Appleoddity
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