Why don't some files compress very much?

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Some files don't compress much

Some types of files compress better than others. Certain data files, such as text files, picture files in the BMP format, and some text style database files can often be compressed by 90% or more. Some other types of files, such as program files, may compress by 50% or so. Learn how to make a zip file.

There also are other types of files that often will not compress well. For example, most multimedia files will not compress much, as they already exist in a highly compressed state. These file types usually employ efficient techniques to compress the data they contain. Examples include but are not limited to, graphic (picture) files (GIF, JPG, PNG, etc), music files (MP3, WMA, etc), and movie files (AVI, MPG, etc). Files that are compressed, as these examples are, usually cannot be compressed more to any significant extent. Therefore, such files don't get much, if any, smaller when they are added to a Zip file (.zip or .zipx).

You would see a similar lack of compression if you zipped some files using maximum compression and then added that Zip file to another Zip file. This second Zip file would not be substantially smaller than the first one (it may even be slightly larger). Again, this is because the data in the original Zip file is already compressed. Some document files, including those created by Microsoft Office, are actually Zip files with a custom file extension.

There are other file types that don't compress well. For example, certain types of encrypted data files, such as those used by home finance programs and some spreadsheet products, will not compress much.

Again, if you create Zip files and see files that cannot be significantly compressed, it is probably because they already contain compressed data or they are encrypted.

If you would like to share a file or some files that do not compress well, you might:

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