EPUB vs MOBI: Is the MOBI Format Finally Dead?

If you've searched "epub vs mobi" and landed here still confused, you're not alone. Half the advice online tells you you need a MOBI file. The other half says MOBI is dead. Both used to be true at different points, which is exactly the problem: most of what's written about MOBI and EPUB was accurate the day it was published and stale within a year of it going live.

So here's the short answer, up front, before we get into the detail: MOBI is dead for uploading. As of 2026, it does not accept new or updated MOBI files anywhere in the pipeline, not for reflowable books, and not for fixed-layout books either, following a final cutoff in March 2025. If you're publishing or updating a book today, you need EPUB (or, for certain workflows, KPF). Nothing in this article is going to tell you to hang onto a MOBI file "just in case." There isn't a case anymore.

This piece walks through exactly when and why that happened, what it wants from you now, what it expects, how the different formats actually compare, why accessibility rules now make EPUB the safer long-term choice regardless of which market you're selling into, and what to do if you're still sitting on a hard drive full of old MOBI files from 2018. It's long, deliberately, because the short version keeps getting copied into forum threads without the nuance, and that's exactly how the confusion perpetuates in the first place: someone reads a headline, skips the dates, and repeats advice that was correct in 2019 as though it still applies today.

A brief history of MOBI

MOBI started life as the format, developed by a French company in the late 1990s for early PDA-based ebook readers. bought outright in 2005, a couple of years before the first even launched, and folded the format into what became the ecosystem. That's the origin of the confusion that still lingers today: MOBI wasn't built from scratch; it was acquired and repurposed, which is part of why it never quite kept pace with formats that were designed collaboratively across the industry.

Being proprietary and built around one company's hardware meant MOBI evolved slowly. It has no real support for modern CSS, handles embedded fonts inconsistently, and carries weak accessibility metadata compared with what's expected of an ebook file today. For a long time none of that mattered much, because if you wanted to sell on, MOBI was simply the price of entry. That's no longer true, and hasn't been for several years now.

What EPUB actually is

EPUB is the open standard maintained by the World Wide Web Consortium, built on the same underlying technologies, HTML, CSS, and XML, that power the rest of the web. Because it's an open standard rather than something owned by a single retailer, virtually every other platform adopted it: most library lending apps, and every wide-distribution aggregator worth using. was the one long-standing holdout, insisting on its own MOBI-based pipeline years after everyone else had standardised around EPUB.

EPUB3, the current major version, adds proper support for embedded audio and video, richer CSS layout control, and accessibility features that align with modern web standards, things like semantic markup that screen readers can actually interpret correctly. EPUB2 still exists in the wild and tools generally still accept it, but if you're formatting a new book today, there's no good reason to target anything other than EPUB3.

The core technical differences

The practical difference for you as an author comes down to a handful of things: EPUB gives you reflowable text that adapts properly across devices and font sizes, full CSS styling control, embedded font support so your headings and drop caps actually render the way you designed them, and accessibility features that MOBI simply wasn't built to carry. MOBI does the basics: text on a page, roughly formatted, and not a great deal more.

When people ask about format MOBI or EPUB, the honest answer in 2026 is that there's no real contest left. EPUB has taken over everywhere, including on itself, via a conversion step that happens automatically and invisibly on its side. Don't think of MOBI as "the format" and EPUB as "the alternative." Think of MOBI as a legacy format that has quietly retired, and EPUB as the universal standard that now works on too.

The MOBI Deprecation Timeline: 2020 to 2025

MOBI didn't disappear overnight, and that's precisely why so much confusion persists. It was phased out in four distinct stages spread across roughly five years, and most articles online only ever cover the first one or two.

November 2020: starts steering authors to EPUB

The first public signal came in November 2020, when Amazon began actively recommending EPUB for new and updated titles rather than requiring it outright. At this point, MOBI uploads still worked perfectly well; nothing was disabled, but the direction of travel was already clear to anyone paying attention. Formatting tool vendors like Apple started adjusting their default export settings around this time, nudging authors toward EPUB output even before it became mandatory.

August 2021: reflowable MOBI uploads end

This is the date most authors remember, and even then, most people get the details slightly wrong. Originally announced a hard cutoff of 28 June 2021, then pushed the deadline back by about a month. From 1 August 2021 onward, stopped accepting MOBI files for new or updated reflowable ebooks, the standard category covering the overwhelming majority of self-published titles: novels, memoirs, non-fiction, poetry collections without complex layout requirements. From that date, reflowable submissions had to arrive as EPUB, DOCX, or KPF, with EPUB strongly preferred for anything beyond the simplest manuscript.

This is also the point where a permanent split appeared between what authors upload and what devices actually receive. kept delivering native files to readers' devices behind the scenes, but the input format authors submitted changed for good.

Late 2022: MOBI disappears from

The second wave of change hit sideloading rather than publishing. Sometime around late 2022, the tool for personal documents, reader magnets, and advance reader copies dropped support for adding new MOBI or AZW files to a library altogether. EPUB became fully supported for sideloading at the same time, converted automatically into its internal reading format on delivery. This is the change that competitor articles miss most consistently, because most of the "MOBI is dead" coverage focused entirely on the publishing side and never circled back to sideloading at all.

18 March 2025: the fixed-layout cutoff

The final stage, and the one that closes the loop entirely: on 18 March 2025, stopped accepting MOBI for fixed-layout ebooks too. Fixed-layout had been the last legitimate corner where MOBI still had a job to do: picture books, heavily illustrated non-fiction, poetry with deliberate line breaks, comics and manga formatted for a specific page. After this date, fixed-layout titles need EPUB or KPF, exactly the same as reflowable ones.

That date is the one competing articles almost always miss, because most of them were written in 2017, 2020, or 2021, years before fixed-layout MOBI was even on the chopping block. If you've read a guide that says "MOBI still works for picture books," check the publish date on it. It's out of date, full stop.

What Accepts for Uploads in 2026

As things stand right now, its format rules are genuinely simple, even though it took five years of incremental changes to get here:

• Reflowable ebooks: EPUB, DOCX, or KPF. No MOBI.

• Fixed-layout ebooks: EPUB or KPF. No MOBI, as of the March 2025 cutoff.

• Print interiors: still PDF, which hasn't changed and sits outside this comparison entirely.

Reflowable ebooks

DOCX is technically accepted for reflowable books, whether you're formatting a psychological thriller or a memoir, but treat that as a fallback rather than a plan. Files convert unpredictably once you introduce anything beyond plain paragraphs, images, tables, drop caps, and custom styling; all tend to shift, resize, or break outright during its automated conversion. EPUB gives you far more control over the finished product, and it's what every professional formatting workflow is built around for exactly that reason.

Fixed-layout ebooks

Fixed-layout is a smaller category but a fussier one, because every element on the page has to sit exactly where you put it. EPUB3's fixed-layout specification handles this properly, preserving background images, text overlays, and precise positioning. KPF, generated through, does the same job through its own tooling.

Print interiors

Worth repeating clearly, because it's a common point of confusion: none of this changes how you handle print. Still want a print-ready PDF interior for paperback and hardback editions, produced through an entirely separate export process from your ebook file. If you're publishing both formats, you'll end up with one EPUB (for the ebook) and one PDF (for print), never a MOBI in sight.

Tools that produce -ready files

If you want its own native format with enhanced typesetting, it produces KPF files directly and is free to use, though it locks you into 's ecosystem. For a properly built EPUB that you can also use for wide distribution, and both handle the heavy lifting with minimal manual intervention; runs in-browser and works across platforms, while is -only but produces exceptionally clean output. is worth knowing about even if you don't use it as your main tool, because it lets you open an EPUB and edit the underlying code directly when something needs a manual fix that your main formatting tool can't handle.

If you're planning to publish beyond as well, it's worth setting up alongside whichever formatting tool you choose. It distributes a single EPUB (or a manuscript it converts for you) out to a long list of other retailers and library platforms from one dashboard, which is precisely the kind of workflow that MOBI's retirement has made simpler: one master file, prepared once, going everywhere, rather than a separate -specific version to maintain alongside it.

Sideloading in 2026

It is a tool for getting personal documents, reader magnets, and advance reader copies onto a platform without going through the store. It's changed as much as 's publishing side has, and it's the part of this story that most guides skip entirely, which is exactly why it's worth its own section here.

What's supported now

Currently supported formats include EPUB, DOCX, PDF, TXT, RTF, HTML, and a handful of common image formats. MOBI isn't on that list. You can't add new MOBI files to a library anymore, whether through the website, the app, email delivery, or USB transfer via the newer workflow, though books added to a library before the change generally remain readable.

What happens to your EPUB after you send it

When you send an EPUB through, it converts it automatically to its internal reading format, AZW3, sometimes referred to as KFX depending on the device and app version, on delivery. You never see that conversion happen; it's invisible to you as the sender. Critically, it preserves reflowable text, embedded fonts, and most of your styling choices, which wasn't always guaranteed with older conversion pipelines. The upshot for authors: if you're distributing reader magnets or ARCs, send EPUB. Not MOBI. That's the current workflow, full stop, whatever an older guide told you two or three years ago.

Legacy MOBI files already in your library

If you sideloaded MOBI files before the cutoff, they typically still open and read fine, hasn't gone back and stripped existing libraries. The restriction only applies going forward, to new files you try to add. If you need to refresh, update, or reformat a book that's already sitting in someone's library as an old MOBI file, the practical move is converting a new EPUB version and sending that instead, rather than trying to overwrite the existing file directly.

EPUB vs MOBI vs KPF vs AZW3: A Feature Comparison

These four terms get mixed up constantly, partly because it has never been especially clear about which ones are upload formats and which are reading formats. Here's how they actually compare:

Reading the table correctly

A couple of clarifications worth having, because a table can't carry all the nuance on its own. AZW3, also called KF8, is a reading format that lives on the device itself, not something you ever upload; it's generated automatically from your EPUB during the publishing process. If you see "AZW3" mentioned in the context of what you need to submit anywhere, that's a sign you're reading outdated or simply incorrect advice.

KPF is its own output format. It's accepted for both reflowable and fixed-layout books, and it can produce excellent typesetting quickly, but it only exists on its platform; there's no equivalent way to open a KPF file on or. If you have any intention of publishing wide, EPUB is the format to build your master file around, using KPF only as an -specific extra if you want it.

Always double-check current acceptance status on their own help pages before you rely on any of this for a live upload. Requirements have already shifted multiple times since 2020, and there's no particular reason to assume they've finished shifting.

The European Accessibility Act, in plain terms

The European Accessibility Act came into force on 28 June 2025, and it requires ebooks sold into the EU to meet accessibility standards, generally interpreted as WCAG 2.1 Level AA, the same benchmark used across the wider web. UK-based authors sometimes assume this doesn't touch them because the UK isn't in the EU, but the law applies to anyone selling into EU markets, which covers most self-published authors distributing through retailers, or the moment those retailers make a title available to EU customers. There's an exemption for micro-enterprises, businesses with fewer than ten employees and under two million euros in annual turnover, which covers the vast majority of solo indie authors, but retailers themselves are also on the hook for compliance, and several have been tightening their own automated checks as a result. In practice, that means EPUBs with clean structure and accessibility metadata are increasingly likely to sail through retailer checks without friction, while poorly structured files risk flags or delays regardless of whether the author is personally exempt from the law.

Why MOBI was never going to keep up here

This is where MOBI's technical limitations stop being a formatting inconvenience and start being a genuine compliance gap. MOBI has no meaningful support for the semantic structure, navigation landmarks, or accessibility metadata that WCAG 2.1 AA and the EU Accessibility Act expect. EPUB3, by contrast, was built with exactly this kind of structure in mind: proper heading hierarchy, a working table of contents, descriptive alt text for images, and a declared reading order that assistive technology can actually follow. None of that retrofits cleanly onto MOBI. It's another reason the format's retirement lines up with where the wider industry was already heading, rather than being an arbitrary decision on its part.

What this means practically

If you're formatting a new book, build accessibility in from the start rather than treating it as an afterthought: proper heading structure throughout the manuscript, meaningful alt text on every image rather than a placeholder, and a clean, working table of contents. Tools like these offer free automated checks against WCAG criteria if you want to verify a file before it goes anywhere near. It's a small amount of extra care at the formatting stage that avoids a considerably more expensive remediation job later, particularly if you're planning to keep a backlist title on sale in EU markets for years to come.

How to Convert an Old MOBI File to EPUB

If you've got legacy MOBI files sitting in a folder somewhere and need to get them back into a format you'll accept, it's the tool for the job- free, actively maintained, cross-platform, and reliable for the majority of straightforward books.

Step-by-step conversion

1. Download from its official website. It's available for Windows and Mac, and installation takes a couple of minutes.

2. Add your MOBI file by dragging it into the library window, or using the "Add books" button in the toolbar.

3. Select the book in your library once it's imported.

4. Click "Convert books," then choose "Convert individually" for a single title, or "Convert books in bulk" if you're processing several files at once.

5. Set the output format to EPUB using the dropdown in the top-right corner of the conversion dialog.

6. Review the conversion settings before you proceed. For most straightforward books, the defaults work fine. If your manuscript has complex formatting, drop caps, pull quotes, embedded images with specific placement, check the "Look & feel" and "EPUB output" tabs and adjust as needed rather than trusting the defaults blindly.

7. Click OK to start the conversion. builds the EPUB and drops it into your library folder, usually within a few seconds for a text-heavy book.

8. Open the result and check it properly, page by page if the book is short enough, before you go anywhere near the upload button.

When to re-export from source instead

One honest caveat worth taking seriously: if you still have the original or file the MOBI was created from, re-export directly from that source instead of converting the old MOBI. Going MOBI-to-EPUB can introduce glitches with images, tables, and embedded fonts that a fresh export from the original manuscript avoids entirely, because you're not asking to reverse-engineer formatting decisions from an already-compressed file. Treat MOBI-to-EPUB conversion as a rescue option for when the source file is genuinely gone, not as your first choice when better alternatives exist.

Quick rescue checklist

If you're dealing with several old files at once, work through them in this order:

• Locate the original source file, if you still have one, before doing anything else

• If the source is gone, convert the MOBI with

• Check that images, tables, and embedded fonts survived the conversion intact

• Validate the result properly in

• Re-upload the finished EPUB to

• Test the finished file on more than one device before calling the job done

Why Your EPUB Might Still Get Rejected

Switching to EPUB solves the format problem, but it doesn't automatically guarantee a smooth upload. A properly built EPUB can still bounce back from or fail checks for reasons that have nothing to do with the MOBI-to-EPUB transition at all. Knowing the common causes saves a lot of frustrated re-uploading.

Validation errors inside the file itself

and its own upload checker both run validation against the EPUB specification, and a file exported from an older or less careful tool can fail on things like malformed HTML inside the EPUB's internal files, missing or broken internal links, or an incomplete manifest. This is far more common with files converted from MOBI through using default settings on a book with unusual internal structure than it is with EPUBs generated fresh by dedicated tools like Calibre, which is one more reason to prefer a fresh export over a conversion wherever you can.

Embedded font licensing

EPUB supports embedded fonts properly, which is a genuine improvement over MOBI, but it comes with a catch authors sometimes miss: not every font licence permits embedding. And similarly, licensed open fonts are generally fine, but a font purchased from a type foundry or bundled with design software often restricts embedding rights specifically. Check your font's licence before baking it into the file, because a rejected upload over licensing is an easy problem to avoid and an annoying one to untangle after the fact.

Cover image and metadata mismatches

Checks that your cover image meets its size and resolution requirements, and that your file's internal metadata- title, author, language, ISBN if you have one- matches what you've entered in the dashboard itself. A mismatch here, even a small one like a subtitle that doesn't quite match, can trigger a rejection or a manual review delay that has nothing to do with the EPUB's core structure being wrong.

The fix, in short

Run every file through before uploading, not after a rejection. It catches the majority of these issues locally, for free, before its own system ever sees the file. For anything that doesn't flag but still rejects, the error message it returns is usually specific enough to point you at the actual problem; read it properly rather than assuming it's a generic MOBI-related issue, because by 2026 it almost certainly isn't.

Which Format Should You Actually Use?

Since "epub or mobi kindle" is still one of the most common questions authors type into, here's the decision laid out in plain terms rather than a flowchart you have to click through:

• Publishing a straightforward reflowable novel or non-fiction book through only? Upload EPUB.

• Publishing a picture book, heavily illustrated title, or anything fixed-layout? EPUB or KPF; MOBI is off the table entirely since March 2025.

• Want native typesetting tools and don't need wide distribution? KPF via is a solid choice.

• Sending an ARC or reader magnet to a? EPUB, sent through.

• Publishing wide, beyond, to or? A single EPUB master file, full stop. It works everywhere. And similar aggregators expect EPUB, or a manuscript file they'll convert on your behalf, not MOBI under any circumstances.

• Sitting on an old MOBI file with no source manuscript left? Convert it first, then treat the result as your working EPUB from that point onward.

Notice that MOBI doesn't come up as the correct answer to a single one of these scenarios. That's really the whole point of this article, condensed into one list.

Common Myths About MOBI and Debunked

A few claims still circulate persistently in forums, groups, and outdated blog posts, so let's deal with each one directly, using its own documentation as the backup rather than secondhand summaries of it.

• "You still need MOBI for." False. Stopped taking MOBI for reflowable books in August 2021 and for fixed-layout books in March 2025. EPUB is the recommended, and effectively required, format for both categories now.

• "MOBI is required for sideloading via." False. supports EPUB directly and hasn't accepted new MOBI files since late 2022.

• "AZW3 and MOBI are basically the same thing." False. AZW3, also known as KF8, is a considerably newer and more capable reading format with proper CSS and font support that MOBI never had. It's generated automatically from your EPUB; you never upload or create it yourself.

• "Fixed-layout ebooks are the one exception that still needs MOBI." False, and this is the myth that trips up the most experienced authors, precisely because it used to be entirely accurate advice. Since 18 March 2025, it simply isn't any more, and continuing to repeat it just spreads the confusion further.

• "Converting MOBI to EPUB always wrecks your formatting." Not necessarily. handles most reflowable books cleanly with minimal fuss. Complex layouts sometimes need manual tidying afterwards, which is exactly why keeping your original source file, wherever possible, matters more than the conversion tool itself.

• "EPUB and MOBI are basically interchangeable, so it doesn't matter which one you keep around." False, and this one causes real problems for authors managing a backlist. MOBI can't carry the accessibility metadata that EU retailers increasingly expect; it locks your book to Amazon's ecosystem alone, and it simply isn't accepted anywhere in the current upload pipeline. Standardising on EPUB as your one working file isn't a preference; it's the only version of your book that every current workflow actually accepts.

What This Means for Authors , and How We Can Help

The practical upshot of five years of incremental change is, thankfully, simpler than the timeline above makes it look: you no longer need to maintain separate MOBI and EPUB versions of anything. One EPUB master file now covers every other major retailer worth being on. That's a genuine simplification for wide-distribution authors who used to juggle two, sometimes three, file types per title, and it removes an entire category of mistake that used to be common: accidentally sending readers an outdated MOBI reader magnet while the EPUB on your website had already been updated with corrections.

Wide distribution gets simpler

If you're publishing beyond, through, direct relationships with, or a library aggregator, you were probably already using EPUB for those platforms and MOBI only for. That split no longer exists. One properly built EPUB now does the whole job, which cuts down on file management, version confusion, and the risk of accidentally uploading an outdated file to one retailer while another has the corrected version.

Fixed-layout and design-heavy books

Fixed-layout titles, picture books, illustrated non-fiction- anything where the visuals carry as much weight as the text- lean heavily on how the file is actually designed, which is exactly where dedicated book design work earns its keep. A conversion alone won't fix a layout that wasn't built properly in the first place; format compliance and good design are two separate problems, and solving the first one doesn't automatically solve the second.

Print is a separate conversation

If your interior includes images, our piece on grayscale vs monochrome vs black and white for book printing is worth reading before you finalise anything for print, since none of the EPUB and MOBI changes discussed above touch your print files at all. Still want a print-ready PDF interior, produced through a completely separate export process, which is a job handled by our book printing team when authors would rather not manage it themselves.

How UK Publishing House can help

If all of this sounds like more file-format admin than you want to own yourself, that's entirely fair. It's genuinely fiddly, it's changed several times over the past five years, and it's easy for even experienced, previously published authors to be working from advice that was accurate the last time they checked and simply isn't any more. Our ebook formatting service builds -ready EPUB and KPF files, checks accessibility structure alongside standard validation, and tests everything properly before you ever see the finished file, so none of the rejection reasons covered earlier in this piece end up being your problem to troubleshoot. It sits alongside our wider publishing support for authors who'd rather hand off the technical side of getting a book live entirely and focus on the writing instead.

Final Checklist Before You Upload

• New or updated reflowable ebook? Upload EPUB.

• Fixed-layout ebook? Upload EPUB or KPF, never MOBI, under any circumstances.

• Sending a reader magnet or ARC? Use EPUB.

• Got an old MOBI file? Convert it before trying to reuse it anywhere.

• Always validate the finished EPUB properly before uploading.

• Keep one EPUB master file for all retailers, rather than maintaining separate versions per platform.

• Test on more than one device or reading app; it catches a great deal, but not absolutely everything.