What EPUB Format Is
EPUB is a file format for e-books — the standard way publishers and authors package text, images, and formatting so that e-readers can display them correctly. When you read a book to your Kindle, phone, or tablet, it is usually in EPUB format or a format based on the same technology.
The name stands for Electronic Publication. Think of it like a PDF, but built specifically for reading on screens of different sizes. A PDF locks everything into a fixed page layout — useful for documents that need to look identical everywhere. EPUB does the opposite: it lets the text reflow and resize based on what device you are reading on, so a book looks right whether you are reading on a phone screen or a 10-inch tablet.
EPUB files are actually compressed folders containing HTML (the language web pages use), CSS (styling instructions), and image files, all bundled together. Your e-reader unpacks this folder and renders the book on the fly, the same way a web browser renders a website.
Key Takeaways
- EPUB is the standard format for e-books because it automatically adjusts text size and layout to fit whatever device you are reading on.
- EPUB files are compressed folders containing HTML, images, and styling information, not locked-down pages like a PDF.
- Most e-readers — including Apple Books, Google Play Books, and many library apps — read EPUB files natively without conversion.
- Amazon Kindle uses its own format called AZW, which is based on EPUB technology but not compatible with other readers without conversion.
- You can open EPUB files on your computer using free software like Calibre or Adobe Digital Editions before sending them to a device.
Why EPUB Became the Standard
Before EPUB existed, publishers had to create separate versions of each book for different devices and formats. An author's book might exist as a Kindle file, a Sony Reader file, an Apple file, and a PDF all at once. This was expensive and error-prone.
EPUB was designed to be a single, open format that any device manufacturer could support. Because it is not owned by any single company, publishers can use it without paying licensing fees. This made it the de facto standard across the industry — the format you will encounter most often when downloading books from libraries, independent publishers, or sites like Project Gutenberg.
The format has been updated twice since its introduction in 2007. EPUB 3, the current version, added support for audio, video, and interactive content, though most books still use the simpler EPUB 2 structure.
How EPUB Files Work on Different Devices
When you open an EPUB file on an e-reader, the device reads the HTML and CSS instructions inside and renders the text in real time. This is why you can change font size, font family, line spacing, and background color on most e-readers — the device is reinterpreting the same underlying file each time you adjust a setting.
Different devices handle EPUB slightly differently. Apple Books on an iPad will display an EPUB file one way; the same file on a Kobo e-reader will look slightly different because each manufacturer has its own rendering engine. This is usually not noticeable, but occasionally an EPUB with complex formatting will display oddly on certain devices.
Some devices do not support EPUB at all. Amazon Kindle devices and the Kindle app use AZW format instead, which is based on EPUB but modified to work only with Amazon's ecosystem. If you want to read an EPUB file on a Kindle, you will need to convert it first using software like Calibre.
Where to Find EPUB Files
Most public libraries offer EPUB downloads through apps like Libby, OverDrive, or Hoopla. When you borrow a book, you read it in EPUB format and it automatically expires after the loan period ends.
Independent publishers and self-published authors often distribute EPUB files directly through their websites or through platforms like Smashwords and Draft2Digital. Project Gutenberg, a volunteer-run archive of public domain books, offers thousands of titles in EPUB format for free.
Major publishers sell EPUB files through retailers like Google Play Books, Apple Books, and Kobo. You can also purchase DRM-free EPUB files (books without copy protection) from smaller retailers and directly from authors.
DRM and Copy Protection in EPUB Files
Many EPUB files sold through major retailers come with DRM (Digital Rights Management), which is copy protection that ties the book to your account. An EPUB file with DRM can only be opened on devices registered to the account that purchased it.
Library EPUB files almost always have DRM. When you read a book from Libby, the DRM automatically expires on the due date, which is why the book stops working after the loan period ends.
Some publishers and retailers sell DRM-free EPUB files, meaning you can copy them to any device and share them freely (though sharing copyrighted books is still illegal). If you want to avoid DRM restrictions, look for publishers that explicitly advertise DRM-free books, or check Project Gutenberg and similar archives for public domain titles.
Opening and Converting EPUB Files
On a computer, you can open EPUB files with free software like Calibre or Adobe Digital Editions. Calibre is particularly useful because it can also convert EPUB files to other formats — if you want to read an EPUB on a Kindle, Calibre can convert it to AZW format.
On a phone or tablet, read an e-reader app that supports EPUB. Apple Books (built into iPhones and iPads) reads EPUB natively. On Android, apps like Google Play Books, Libby, and Moon+ Reader all support EPUB.
If you have an EPUB file with DRM protection, you will need to open it with the app or device it was purchased for. You cannot straightforward move a DRM-protected EPUB from one device to another without removing the protection first, which requires software and may violate your retailer's terms of service.
EPUB vs. Other E-Book Formats
EPUB is the most common format, but it is not the only one. Here is how it compares to formats you might encounter:
PDF is a fixed-layout format where text and images are locked into place. PDFs are useful for documents that need to look identical on every screen, but they are poor for reading on phones because the text does not resize. Many academic papers and textbooks are distributed as PDFs.
AZW is Amazon's proprietary format, used on Kindle devices and the Kindle app. It is based on EPUB technology but only works within Amazon's ecosystem. If you buy a book on Amazon, you get an AZW file, not an EPUB.
MOBI is an older format that Kindle used before switching to AZW. You will rarely encounter MOBI files today, but Calibre can still convert them if needed.
HTML is sometimes used for straightforward e-books, especially self-published works. An HTML file is just a web page saved to your device — it works on any device with a browser, but it does not have the specialized features of EPUB.
Frequently Asked Questions
Can I read an EPUB file on my Kindle?
Not directly — Kindle devices and the Kindle app only read AZW format. You can convert an EPUB to AZW using free software like Calibre, then transfer the converted file to your Kindle. Some EPUB files with DRM protection cannot be converted without removing the copy protection first.
What happens if I read an EPUB with DRM and then lose access to my account?
You will no longer be able to open the file. DRM-protected EPUB files are tied to the account that purchased them. If you lose access to that account or the retailer shuts down, you lose access to the book. This is why some readers prefer DRM-free books.
Is EPUB the same as a PDF?
No. EPUB files reflow text to fit your screen size, so you can change font size and the text adjusts automatically. PDFs have fixed layouts where text and images stay in the same position, which is why PDFs are hard to read on small screens. EPUB is designed for e-books; PDF is designed for documents.
Can I edit an EPUB file myself?
Yes, because EPUB files are just compressed folders containing HTML and images. You can extract the contents, edit the HTML and images, and repackage it. Calibre has a built-in editor for this. However, editing a DRM-protected EPUB requires removing the protection first.
Where can I get free EPUB books?
Project Gutenberg offers over 70,000 public domain books in EPUB format. Your local library also offers free EPUB downloads through apps like Libby and OverDrive. Standard Ebooks is a volunteer project that offers high-quality, formatted versions of public domain books in EPUB.