File Extension File Type Popularity
EPUB Open EBook Format Very popular ( 5 / 5 )

EPUB file structure

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EPUB files is basically a ZIP archive. E-book readers can access such files without unpacking them. However, some interesting information can be obtained by unpacking and viewing an EPUB archive.




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EPUB standard clearly describes the required elements of any EPUB file:

  1. OPS folder

  2. Mimetype file

  3. Container.xml file



OPS folder holds the book contents, such as:

  • fonts used in the publication

  • illustrations (image files)

  • formatting file (CSS stylesheet)

  • text file (HTML)

  • content.OPF file

  • toc.NCX file


OPF file is commonly referred to as content file as it contains e-book metadata. It is an XML-formatted file that describes the format of the publication (Package section). It is further divided into the following elements:

  • metadata – describes such properties as: title, ISBN, or language

  • manifest – lists all the files comprising the publication

  • spine – contains references to publication contents that are listed in the table of contents (must point to a proper toc.NCX file)

  • guide – additional content such as bibliography or list of figures.

toc.NCX file is an XML-formatted table of contents.

Mimetype file contains information necessary to identify given publication. This file is used as a base for validating given publication.

Container.xml file stored in META-INF folder point to the location of content.OPF file.