What Is the EPUB File Format?

Official logo of the EPUB format. IDPF

Question: What is EPUB?

Answer: First released in 2007, EPUB is actually short for “electronic publication,” which is an open standard format for e-books. For the uninitiated, there are actually a wide variety of e-book formats out there, which I detail in my big eBook File Compatibility Chart. These include popular formats such as PDF and TXT as well as some more obscure-sounding files such as PRC and PP2.

As an open standard, EPUB has great compatibility across a wide range of e-readers and even became the de facto universal standard within the e-reader sector for most devices not named “Kindle.” That’s because Amazon’s AZW format for its Kindle e-reader line is a proprietary or “closed” format, which means AZW ebooks can not be read by other e-readers. Interestingly, Amazon's Kindle line also does not support EPUB natively. The plot thickens, hmm? The Kindle is actually the only major e-reading device that does not support EPUB. A large part of it likely has to do with pushing its own preferred format and sales of e-books from its own store as well. Before they got discontinued, Sony’s e-readers,  supported PUB. Barnes & Noble’s Nook  and Kobo's  Glo HD and Aura  H2O also still support EPUB, as does Google Books.

As an e-book format, EPUB allows for popular e-reading features such as reflowing and resizing text as well as word highlighting and notes.

EPUB files are usually pretty easy to open, not just by e-readers but devices such as iPhones, iPads, Android devices and PCs. There are a wide range of programs that can open EPUB files, for example. The fact that they can act like ZIP files also make EPUB documents even easier to work with. The files can even be converted into other file formats with the use of conversion programs.

Its open nature and wide-range adoption also makes the format an ideal candidate for sideloading to an e-reader or other device if you so wish.

At the same time, EPUB files also can be like MP3 files in terms of security. See, like the popular music file format, EPUB files can also have “digital rights management” included by publishers who want to restrict the copying of such files. Bookstores for Kobo and Barnes & Noble, for example, use copy protection to prevent rampant sharing of their e-books by consumers.

With the advent of smartphones and tablets, the momentum for traditional e-readers have slowed down in general, but EPUB’s ability to work with such devices means it isn’t dead (though the format itself hasn’t been updated since about 2014). Meanwhile, the Kindle continues to dominate in markets such as the United States. Although the Kindle doesn’t support EPUB, however, can still display the file via conversion to compatible formats such as MOBI. In addition to Amazon’s own KindleGen tool, which can convert EPUB files, another popular option is Calibre, which is a popular e-book management tool that lets you convert files should the need arise.

Despite not being natively supported by Amazon, EPUB continues to be popular among e-book fans because it allows them to acquire a wide array of free content.

By using sources such as Open Library and Project Gutenberg, for example, users can access thousands of free e-books that they can read to their hearts’ content. Even as Amazon continues to hold off on direct support for the format, the lack of support from the reigning market leader among e-readers has failed to kill off the popular format. Chalk one up to the allure of openness among savvy readers.

Jason Hidalgo is About.com’s Portable Electronics expert. Yes, he is easily amused. Follow him on Twitter @jasonhidalgo and be amused, too. For more articles about e-readers, check out the Portable Electronics E-Reader hub