EPUB MOBI Format in 2026: Which Is Best for Authors?

TL;DR

EPUB is the universal ebook standard that works everywhere, including Amazon. MOBI is dead: Amazon stopped accepting it in 2021, and Send-to-Kindle dropped it entirely by August 2022. When you upload an EPUB to Kindle Direct Publishing, Amazon automatically converts it to its own KFX format. The only ebook file formats self-publishing authors need today are EPUB for ebooks and PDF for print-ready or complex layout files.

Why Ebook Formats Confuse Authors

Self-publishing throws an alphabet soup at you: EPUB, MOBI, AZW, AZW3, KFX, KPF, PDF. Some of these formats are open standards. Some are proprietary to Amazon. And some are flat-out obsolete, even though outdated guides still recommend them.

The confusion isn’t harmless. Practitioners on indie author forums regularly report books arriving with broken formatting, missing tables of contents, or chapters that appear in random order on Kindle devices. These problems almost always trace back to using the wrong source file or converting between formats unnecessarily.

This glossary cuts through all of it. Every term is defined in plain English with practical context for authors who are writing, formatting, and publishing their books in 2026. If a format is dead, it’s labeled dead. If you don’t need to think about it, that’s stated clearly too.

The MOBI Deprecation Timeline: What Happened and When

Before jumping into the full glossary, the single most important thing to understand about the epub mobi format question is that MOBI is no longer a living format. Here’s exactly how it died:

• November 2020: Amazon KDP began recommending EPUB over MOBI for new book uploads.
• June 28, 2021: KDP officially stopped accepting MOBI, PRC, and AZK files for new reflowable ebooks.
• August 1, 2021: The cutoff took full effect. MOBI uploads were rejected.
• April 2022: Amazon added EPUB support to its Send-to-Kindle feature, eliminating MOBI’s last practical advantage for personal document delivery.
• August 2022: MOBI files sent via Send-to-Kindle were silently rejected. They simply never appeared on devices.

No announcement, no error message. The files just vanished. That’s how thoroughly Amazon buried the format it once championed.

There is no practical reason to create new MOBI files in 2026. The one narrow exception involves Print Replica files for heavily illustrated books (think photography collections or picture books) where exact page positioning matters. For everything else, EPUB won.

A-to-Z Glossary of Ebook Formats and Related Terms

ARC (Advance Reader Copy)

An ARC is a pre-release version of your book sent to reviewers, bloggers, and street team members before your official launch date. Authors use platforms like BookFunnel and Book Sprout to distribute ARCs.

For years, sending ARCs meant producing both EPUB and MOBI files because Kindle readers couldn’t open EPUBs. That’s no longer true. Since April 2022, Kindle accepts EPUB files through Send-to-Kindle, so a single EPUB covers every e-reader on the market. Some authors also include a PDF version as a backup for readers who prefer reading on laptops or tablets without dedicated ebook apps.

For more on ARC timing and strategy, see this guide on book launch tips and ARCs.

AZW / AZW3 (Kindle Format 8)

AZW was Amazon’s first proprietary ebook format, built directly on MOBI’s architecture. AZW3, also called Kindle Format 8 (KF8), replaced it with support for HTML5 and CSS3, allowing much richer formatting.

Authors never create AZW or AZW3 files directly. When you upload an EPUB or DOCX to KDP, Amazon’s backend handles the conversion automatically. These formats exist purely on the delivery side, between Amazon’s servers and the reader’s Kindle device.

Calibre

Calibre is a free, open-source ebook management application that can convert between roughly 30 format pairs. Its conversion engine (called ebook-convert) is battle-tested and widely trusted by the indie author community.

The most common use case today is converting old MOBI files to EPUB. But a critical warning applies: every format conversion is lossy. Each round trip through a converter strips or alters some formatting. Converting an EPUB to MOBI and back to EPUB will produce a degraded file. If your source format is already correct, don’t re-convert it “for safety.” That makes things worse, not better.

DRM (Digital Rights Management)

DRM is encryption technology that restricts how ebook files can be copied, shared, or transferred between devices. Both EPUB and MOBI formats support DRM, as do Amazon’s proprietary formats.

The key thing authors should know: DRM-protected files cannot be freely converted between formats. A DRM-locked MOBI can’t be converted to EPUB without the rights holder’s authorization. Most retail platforms apply DRM automatically, and authors can usually choose whether to enable or disable it during the publishing process. Opinions in the indie community are split. Some authors view DRM as essential piracy protection, while others argue it frustrates legitimate customers without stopping determined pirates.

Enhanced Typesetting

Enhanced Typesetting is Amazon’s proprietary rendering engine, built on the KFX format. It automatically applies hyphenation, kerning, ligatures, drop caps, and Amazon’s custom Bookerly font to create a better reading experience on Kindle devices.

This feature is applied automatically when authors upload properly formatted EPUB or DOCX files to KDP. There’s nothing extra to enable. If your source file is well-structured, Enhanced Typesetting just works. If your file has messy HTML or inconsistent styles, the results can be unpredictable. Our Kindle formatting glossary covers the specifics of getting this right.

EPUB (Electronic Publication)

EPUB is the ebook industry’s universal standard. It is the format self-publishing authors should care most about.

Maintained by the W3C (World Wide Web Consortium), EPUB is an open standard built on the same technologies that power websites: HTML, CSS, and XML. It works on Apple Books, Kobo, Barnes & Noble Nook, Google Play Books, and, since 2021, Amazon KDP.

The adoption numbers tell the story. In 2023, 85% of new ebook releases used the EPUB3 format. With Amazon controlling roughly 83% of U.S. ebook sales (including Kindle Unlimited), and Amazon now accepting EPUB uploads, this single format covers virtually the entire market.

If you’re publishing wide across Amazon and other platforms, EPUB is the one format you need.

EPUB 3 / EPUB 3.3

EPUB 3.3 became an official W3C Recommendation on May 25, 2023. It’s the current version of the standard, and it’s backward compatible with EPUB 3.2.

What EPUB 3 added over the older EPUB 2: multimedia content (embedded audio and video), improved accessibility features, better support for global typography (including right-to-left languages and vertical text), and richer metadata. For most novels and standard nonfiction, the differences between EPUB 2 and EPUB 3 are invisible to readers. But for textbooks, interactive content, or books distributed to accessibility-focused libraries, EPUB 3 compliance matters.

EPUBCheck (Validation)

EPUBCheck is the official validation tool for verifying that an EPUB file meets the W3C specification. Running your file through EPUBCheck before uploading to retail platforms is one of the simplest ways to avoid rejection.

Common errors it catches include malformed HTML, missing metadata fields, broken internal links, and images referenced but not included in the package. Retailers like Apple Books are particularly strict about validation. A file that passes EPUBCheck will almost always pass retailer QC. This tool is free and available as both a command-line utility and through web-based interfaces.

Fixed Layout vs. Reflowable

These are the two fundamental approaches to ebook page structure.

Reflowable text adapts to the reader’s screen size and font preferences. The words reflow to fit whether someone is reading on a phone, a Kindle, or a tablet. This is the standard for novels, memoirs, business books, and most nonfiction.

Fixed layout preserves the exact positioning of text and images on each page. It’s required for children’s picture books, comics, cookbooks with complex layouts, and any content where spatial relationships between elements matter. Fixed-layout ebooks don’t allow readers to change font size or reflow text.

Most ebooks are reflowable. If yours is fixed layout, the production process is significantly more involved.

KDP (Kindle Direct Publishing)

KDP is Amazon’s self-publishing platform. It accepts EPUB, DOCX, and KPF file uploads for ebooks. Internally, KDP converts these source files to AZW3 or KFX for delivery to Kindle devices and apps.

The key takeaway: you never need to worry about Amazon’s internal formats. Your job is to give KDP a clean, well-formatted source file. EPUB is the best option. Amazon’s system handles everything from there.

For a complete walkthrough, see this guide to self-publishing on Amazon KDP.

KFX (Kindle Format X)

KFX is Amazon’s newest and most advanced ebook delivery format, introduced in 2015. It powers Enhanced Typesetting and offers improved text rendering, smaller file sizes, and the best reading experience on modern Kindle devices.

Authors don’t create KFX files. Amazon generates them automatically from uploaded EPUB or DOCX source files. Think of KFX as the final compiled output, not something you interact with directly.

Kindle Create / KPF (Kindle Package Format)

Kindle Create is Amazon’s free desktop application for formatting manuscripts. It provides a visual editor where authors can apply themes, insert images, and preview how their book will look on Kindle devices.

Kindle Create exports files in KPF format (Kindle Package Format). When you upload a KPF to KDP, Amazon converts it to KFX during the publishing process. KPF is an alternative to uploading EPUB for authors who want Amazon’s own formatting tool rather than using third-party software like Vellum or Atticus.

Kindle Previewer

Kindle Previewer is a free Amazon desktop application that simulates how your ebook will appear across different Kindle devices and screen sizes. It accepts EPUB files and renders a KFX-equivalent preview.

This is a useful quality-assurance step before hitting “publish” on KDP. It won’t catch every issue, but it reveals major formatting problems like broken images, misaligned text, and navigation errors.

Metadata (OPF)

Metadata is the structured information embedded inside your ebook file: title, author name, description, language, publisher, ISBN, and keywords. In the EPUB format, this metadata lives in an OPF (Open Packaging Format) file within the ebook’s package.

Metadata matters because it directly affects discoverability. When retail platforms index your book, they pull from this embedded metadata. Incomplete or inaccurate metadata means your book may not appear in the right categories or search results. If you need an ISBN, our guide on what ISBNs are and how to get one covers the essentials.

MOBI (Mobipocket)

MOBI was created in 2000 by Mobipocket, a French company. Amazon acquired Mobipocket in 2005 and adopted MOBI as the foundation for its Kindle e-readers. For over a decade, MOBI was the only format Amazon accepted.

Its technical limitations became increasingly painful as devices improved. Image sizes were capped at 64KB, formatting options were limited compared to EPUB, and the format couldn’t support multimedia content. MOBI did support bookmarks and annotations, but that wasn’t enough to justify its continued existence.

As detailed in the deprecation timeline above, KDP stopped accepting MOBI for reflowable ebooks in June 2021. Send-to-Kindle dropped MOBI support entirely by August 2022. The epub mobi format debate is settled: MOBI is a legacy artifact.

If you still have old MOBI files from previous publishing efforts, they’ll continue to function on older Kindle devices. But for anything new, EPUB is the path forward.

PDF (Portable Document Format)

PDF is the only common ebook file format that maintains a truly fixed layout, exactly preserving how graphics, text, and images appear on the page. It’s an open ISO standard and is universally readable.

PDF is still the best choice for print-ready files, textbooks with complex layouts, manuals, sheet music, and graphically detailed documents. However, PDFs are not ideal for e-reader devices because the text doesn’t reflow. Reading a PDF on a small Kindle screen means constant zooming and scrolling.

For ebook distribution, use EPUB. For print production and complex visual layouts, use PDF. Most authors end up needing both.

Reader Magnet

A reader magnet is a free ebook offered by an author (usually through their website or a service like BookFunnel) to attract newsletter subscribers and build an email list. It’s one of the most effective tools for growing a direct readership.

The format question used to be a headache. Authors needed both an EPUB and a MOBI version to cover all e-reader types. Now that Kindle accepts EPUB, a single EPUB file (plus an optional PDF backup) covers every device. This simplifies production and reduces the friction of setting up a new reader magnet.

Reflowable Text

Reflowable text automatically adjusts to fit the screen size and accommodates the reader’s chosen font, font size, and margin preferences. It’s the default and expected behavior for the vast majority of ebooks, including all standard fiction and most nonfiction.

If your book is a novel, a memoir, a business book, or any primarily text-based work, your ebook should be reflowable. Fixed layout should only be used when spatial positioning of visual elements is critical to the reading experience.

Send to Kindle

Send to Kindle is Amazon’s feature that lets users email or upload personal documents directly to their Kindle device or app. It has accepted EPUB files since April 2022.

This matters for authors because it’s how many readers get ARCs, reader magnets, and personal ebook purchases from non-Amazon sources onto their Kindle. The fact that Send to Kindle now accepts EPUB and rejects MOBI is one of the clearest signals that the epub mobi format transition is complete.

Sideloading

Sideloading means manually transferring ebook files to a device, typically via USB cable or email, rather than purchasing through a retail store. Modern Kindle devices accept sideloaded EPUB files, a significant change from the MOBI-only era.

Sideloading is common among readers who use Calibre to manage their personal libraries, readers who receive ARCs from authors, and anyone who purchases DRM-free ebooks directly from publisher websites.

W3C / IDPF

The W3C (World Wide Web Consortium) is the international standards body that maintains the EPUB specification. The EPUB standard was previously maintained by the IDPF (International Digital Publishing Forum), which merged into the W3C in 2017.

This merger was significant because it placed ebook standards under the same organizational umbrella as HTML, CSS, and web accessibility standards. It’s a major reason why EPUB 3.3 is so closely aligned with modern web technologies.

Wide Distribution

Wide distribution means publishing your ebook across multiple retail platforms, including Amazon, Apple Books, Kobo, Barnes & Noble, Google Play Books, and library channels, rather than going exclusive to a single store (typically through Amazon’s KDP Select / Kindle Unlimited).

EPUB is the single format needed for wide distribution. Every major non-Amazon retailer accepts EPUB natively, and Amazon now accepts it too. This is one of the strongest practical arguments for making EPUB your primary production format.

EPUB vs. MOBI vs. PDF: Quick Comparison

Feature	EPUB	MOBI	PDF
Status in 2026	Active industry standard	Deprecated	Active
Open standard?	Yes (W3C)	No (Amazon proprietary)	Yes (ISO)
Reflowable text	Yes	Yes	No
Amazon KDP accepts?	Yes	No (since June 2021)	Limited use
Apple / Kobo / B&N?	Yes	No	Varies by platform
Multimedia support	Yes (EPUB 3)	No	Limited
Max image quality	High resolution	64KB cap (historical)	High resolution
Best use case	All ebooks	Nothing new	Print-ready and complex layouts

What Format Do You Actually Need?

All the terminology above boils down to a few simple decisions.

Publishing to Amazon only? Upload an EPUB (or DOCX) to KDP. Amazon converts it to KFX automatically. You don’t need to think about AZW, AZW3, or KFX.

Going wide across multiple retailers? Upload EPUB everywhere. It’s accepted by Amazon, Apple Books, Kobo, Barnes & Noble, and Google Play. One format, universal coverage.

Sending ARCs or reader magnets? EPUB covers every e-reader. Add a PDF as a backup for readers who prefer desktop or tablet viewing without an ebook app.

Have old MOBI files you still need? Convert them to EPUB using Calibre. But do it once and keep the EPUB as your new master file. Don’t convert back and forth, because each conversion degrades the formatting.

Working with complex visual layouts? PDF for print. Fixed-layout EPUB for digital. Talk to a professional formatter if you’re unsure which approach fits your content.

Not sure where to start? Professional formatting services deliver retail-compliant files across every platform, eliminating the guesswork entirely.

Explore book publishing services at Alpaca Authors

The Market Context Behind These Formats

Understanding why these formats matter requires a quick look at the market. Amazon Kindle holds approximately 72% of the e-reader device market. When Kindle Unlimited is included, Amazon controls roughly 83% of U.S. ebook sales. Barnes & Noble Nook accounts for about 10%, with the remaining 18% spread across other apps and platforms.

In the United States, 24% of adults read ebooks as of 2025, compared to 46% who read physical print books and 23% who listen to audiobooks. The global ebook market was valued at $18.2 billion in 2023 and is projected to reach $29.9 billion by 2033.

These numbers explain Amazon’s outsized influence on format standards. When Amazon decided to drop MOBI and accept EPUB, it effectively made EPUB the only ebook format that matters worldwide. The epub mobi format question isn’t a close competition. It’s a settled outcome.

For authors considering audiobook production alongside their ebook, the format ecosystem is entirely different (MP3, M4B, and platform-specific containers). Our audiobook production guide covers that separate world.

Frequently Asked Questions

Do I still need a MOBI file for anything?

No. There is no practical reason to create new MOBI files in 2026. Amazon stopped accepting them for new reflowable ebooks in June 2021, and Send-to-Kindle stopped accepting them in August 2022. The only narrow exception involves Print Replica files for heavily illustrated books, and even those cases are increasingly handled through fixed-layout EPUB or KPF.

What if I previously uploaded MOBI files to KDP?

Books that were published using MOBI files before the June 2021 cutoff continue to function. Amazon doesn’t retroactively remove already-published titles. However, if you update those books, you’ll need to upload an EPUB or DOCX as your new source file.

Can I convert EPUB to MOBI for someone with an old Kindle?

Technically yes, using Calibre. But almost no one needs to. Even older Kindle models from the 2018-2019 era support EPUB through firmware updates or the Send-to-Kindle email feature. The number of active Kindle devices that genuinely require MOBI is vanishingly small.

What’s the difference between EPUB and EPUB 3?

EPUB 3 (current version: 3.3) is the latest iteration of the EPUB standard. It added multimedia support, improved accessibility, global typography, and richer metadata over the older EPUB 2. For standard novels and text-heavy nonfiction, readers won’t notice a difference. For interactive or multimedia-rich content, EPUB 3 is essential.

Should I use Kindle Create or upload my own EPUB?

Either works. Kindle Create is a solid free option that exports to KPF for upload to KDP. Uploading your own EPUB gives you more control and produces a file that works across all retailers, not just Amazon. If you’re going wide, EPUB is the clear choice. If you’re Amazon-exclusive and want a simple visual tool, Kindle Create is fine.

Does converting between formats hurt quality?

Yes. Every conversion is lossy to some degree. Converting MOBI to EPUB and then back to MOBI, for instance, will degrade formatting, especially around images, embedded fonts, and complex CSS styling. The best practice is to create your ebook in EPUB from the start and avoid unnecessary format conversions.

What about audiobook file formats?

Audiobooks use a completely different set of formats (MP3, M4B, FLAC) and have their own production and distribution requirements. They’re unrelated to the epub mobi format question. If you’re interested in audiobook production, Alpaca Authors offers audiobook services covering narrator casting, recording, mastering, and distribution to 45+ platforms.

How do I check if my EPUB file is valid before uploading?

Run it through EPUBCheck, the official W3C validation tool. It’s free and catches the most common errors that cause retail platform rejections: malformed HTML, missing metadata, broken internal links, and packaging errors. Clean EPUBCheck validation is the closest thing to a guarantee that your file will pass retailer QC on the first try.