Ebooks have surged in popularity, now representing 30% of all book sales in the United States due to their affordability and portability.
However, it's not all smooth reading for users. Unlike universal MP3 files that play on any music device, ebooks come in a confusing array of proprietary and open formats. Adding to the challenge, not every e-reader supports every format.
In this expert guide, we break down the most common ebook formats, their strengths and limitations, and which devices support them—drawing from years of testing and managing vast digital libraries.
EPUB stands as the most widely used ebook format. Developed by the International Digital Publishing Forum (now part of the World Wide Web Consortium), it succeeded the older Open eBook (OEB) format in 2007.
As a free, open standard independent of any vendor, EPUB dominates the market. It handles color images, SVG graphics, interactive elements, and even full-length videos—though these features are less common.
Think of it as the MP3 of ebooks: broadly compatible across e-readers and operating systems. Publishers can apply DRM, but it's removable on owned ebooks. The main caveat? Amazon Kindle devices (except the Kindle Fire tablet) don't support it natively. Use Calibre to convert: How to Convert eBooks to Kindle Format When Importing into Your Library. Calibre can auto-convert EPUB to MOBI on import.
MOBI evolved from the OEB format when Mobipocket forked it in 2000 for its reader software. Amazon acquired the company in 2005, supporting it until shutting down the site in 2016—yet the format endures.
Unlike open EPUB, MOBI is proprietary and lacks audio/video support. It's backed by most major e-readers except Barnes & Noble Nook.
Note: MOBI also uses the PRC extension.
Amazon's proprietary AZW (launched with the first Kindle in 2007) and AZW3 (introduced with Kindle Fire in 2011) power Kindle ebooks. How to Download Free Amazon Kindle eBooks on Your PC delivers them in AZW3, which offers superior fonts, styles, and layouts over AZW.
Both build closely on MOBI technology—rumored to stem from Amazon's Mobipocket acquisition. They uniquely support audio and video among Amazon formats.
Limited to Kindle devices natively, though Android, iOS, Calibre, and similar apps handle them. Nook and Kobo do not.
IBA powers books from Apple's iBooks Author. Similar to EPUB but reliant on Apple Books app widgets, it's not cross-compatible.
Exclusive to iBooks Author titles; iTunes Store bestsellers arrive as DRM-protected EPUB. IBA excels with video, audio, images, and interactivity.
PDFs are ubiquitous for ebooks thanks to web familiarity, but they falter on reflow—the ability to adapt layout to screen size and user preferences.
Dedicated formats reflow seamlessly via content stream sequencing. PDFs can use tags for structure, though e-reader support lags. Positively, PDF is an open ISO 32000 standard since 2008.
Lesser-known options occasionally appear...
Sony's Broadband eBook formats for its readers: LRS (now open), LRF, and LRX (proprietary). Sony shifted to EPUB.
Russia-born XML-based FB2 shines for collectors with embedded metadata and easy conversions.
Favored in science for 10x better compression than PDF—over 100 B&W scans per MB. 4 Ways to Compress and Reduce the Size of a PDF File shares PDF tips too.
Microsoft's defunct proprietary format (discontinued 2011) required its Reader app with DRM.
Rich Text Format works universally, surpassing TXT with reflow, formatting, and special characters.
Focus on EPUB, MOBI, and AZW for most needs. Build a multi-device library with EPUB. For Kindle-centric collections, MOBI offers broader compatibility than feature-rich AZW.
For file type deep dives: JPEG, GIF, or PNG? Image File Types Explained and Tested and Video File Types Explained & Compared: AVI, MKV, or MP4?
