An EPUB file is a digital book format that works on phones, tablets, e-readers, and computers
EPUB stands for Electronic Publication. It is a file format — like .pdf or .jpg — designed specifically for books and long documents. When you download an e-book from a library or bookstore, it is usually an EPUB file. The format lets you read the same book on your phone, your tablet, your e-reader, or your computer without converting it first.
EPUB files are text-based, which means they reflow to fit whatever screen you are reading on. If you make the text larger, the words wrap to fill the page. If you read on a phone held sideways, the layout adjusts. A PDF file does not do this — it stays the same size and shape no matter what device opens it.
Most EPUB files are unencrypted and can be opened by any reading app. Some publishers add digital rights management (DRM), a lock that ties the book to one account or device, but this is less common than it used to be.
Key Takeaways
- EPUB files are designed for books and adapt their layout to any screen size, unlike PDFs which stay fixed.
- You can open EPUB files on phones, tablets, e-readers, and computers using free reading apps like Apple Books, Google Play Books, or Calibre.
- Most libraries lend EPUB files through apps like Libby or OverDrive, and you can keep them for two to three weeks before they automatically return.
- EPUB files are smaller than PDFs and use less storage space on your device.
How EPUB files differ from PDFs and other formats
A PDF is a fixed layout. It looks the same on every device because the publisher decided exactly where every word and image goes. This works well for documents like forms, contracts, or textbooks with precise page numbers. It does not work well for reading on a small phone screen — you have to zoom in and scroll around.
An EPUB file is flexible. The text and images flow to fit the screen. You can change the font size, the font itself, the line spacing, and the background color without breaking the layout. Publishers do not control how the book looks on your device — you do.
MOBI and AZW are Amazon's formats, used on Kindle devices and the Kindle app. They work similarly to EPUB but are not as widely supported. Most libraries and bookstores that sell e-books offer EPUB, not MOBI, so EPUB is the more universal choice if you want to read on multiple devices.
What apps and devices can open EPUB files
Any device with an internet connection can read EPUB files. On iPhone and iPad, use the built-in Apple Books app — it comes with iOS and opens EPUB files automatically. On Android phones and tablets, Google Play Books is the standard choice, though other apps like Libby (for library books) and Kindle also support EPUB.
On e-readers, most devices support EPUB. Kobo e-readers are built for EPUB. Amazon Kindles use their own format by default but can read EPUB files if you convert them first. If you want to read EPUB files directly without conversion, a Kobo or other non-Amazon e-reader is simpler.
On Windows and Mac computers, use Calibre (free, open-source software) or Apple Books on Mac. Calibre is the most flexible option — it opens EPUB files, converts between formats, and manages a library of books on your computer.
Where to find EPUB files to read
Your local library is the easiest source. Most libraries lend e-books through apps like Libby (owned by OverDrive) or through their own library app. You search for a book, borrow it for two to three weeks, and it appears in your reading app. When the loan period ends, the book automatically returns — no late fees. Libby works with thousands of libraries across the United States and Canada.
Online bookstores like Smashwords, Draft2Digital, and many independent publishers sell EPUB files directly. You download the file to your computer and then transfer it to your reading app or device. This gives you permanent ownership, unlike a library loan.
Project Gutenberg and Standard Ebooks offer free EPUB files of books in the public domain — older books whose copyright has expired. These are legal to download and share.
How to move an EPUB file between devices
If you download an EPUB file to your computer, you can move it to your phone or tablet by email, cloud storage, or a USB cable. Email the file to yourself, download it on your phone, and tap it to open it in your reading app. Or save it to Google Drive, Dropbox, or iCloud, and open it from there.
If the EPUB file has DRM (a digital lock), you cannot move it between devices. It is tied to the account you used to download it. Most library EPUB files have DRM, which is why you read them through the library's app rather than downloading and moving them around.
Calibre on your computer can manage EPUB files across devices. You can import files, organize them by title and author, convert them to other formats if needed, and sync them to an e-reader or phone.
What to do if an app will not open your EPUB file
If you download an EPUB file and your reading app does not recognize it, the file may be corrupted or the app may not support EPUB. Try opening it in a different app first — if it works in one app but not another, the problem is the app, not the file.
If no app will open it, the file may have been damaged during download. Try downloading it again. If you downloaded it from a bookstore or library, contact their support — they can resend the file or help you troubleshoot.
If you need to convert an EPUB to a different format (like PDF or MOBI), use Calibre. Open Calibre, import the EPUB file, select it, click "Convert books", choose your output format, and click "OK". Calibre will create a new file in the format you chose.
EPUB file size and storage
Most EPUB files are small — typically between 500 kilobytes and 5 megabytes, depending on the length of the book and whether it contains images. A typical novel is around 1 to 2 megabytes. This means you can store hundreds of books on a phone or tablet without running out of space.
PDFs of the same book are often larger because they contain fixed layout information for every page. An EPUB file is more efficient because it only stores the text and images, and lets your device decide how to arrange them.
Frequently Asked Questions
Can I read an EPUB file on a Kindle?
Kindle devices and the Kindle app are designed for Amazon's MOBI and AZW formats, not EPUB. You can convert an EPUB to MOBI using Calibre and then send it to your Kindle, but you cannot open an EPUB file directly on a Kindle device without conversion first.
Do I need an internet connection to read an EPUB file?
No. Once you download an EPUB file to your device, you can read it offline. You only need internet to download the file or to borrow a book from your library app.
Can I edit an EPUB file?
Yes, but it requires special software. EPUB files are actually compressed folders containing HTML, CSS, and image files. You can unzip an EPUB, edit the HTML in a text editor, and rezip it. For simpler edits like changing fonts or colors, most reading apps let you customize how the book looks without editing the file itself.
What happens to my EPUB books if I switch phones?
If your books are stored in a reading app account (like Apple Books or Google Play Books), they sync to your new phone when you sign in with the same account. If you downloaded EPUB files directly to your old phone, you will need to download them again on your new phone or transfer the files using cloud storage.
Is it legal to download EPUB files from websites?
It depends on the source. EPUB files from your library, from publishers, and from sites like Project Gutenberg are legal. EPUB files from sites that distribute copyrighted books without permission are not legal to download. When in doubt, use your library or buy from a legitimate bookstore.