EPUB, AZW3, and MOBI all hold the same book — they just answer to different devices. A modern Kindle wants AZW3, everything else wants EPUB, and MOBI only earns its place on pre-2012 hardware. Here's the reasoning, plus how to switch formats in a browser without installing anything.
At a Glance
If you remember one table, make it this one.
| EPUB | AZW3 | MOBI | |
|---|---|---|---|
| Backed by | Open standard (IDPF) | Amazon (KF8) | Amazon (legacy) |
| Best on | Kobo, Apple Books, most e-readers | All modern Kindles | Pre-2015 Kindles |
| Styling | Full CSS, embedded fonts | Full CSS, embedded fonts | Limited; flattens layout |
| Open or locked | Open, portable | Amazon-centric | Amazon-centric |
| Future | Actively developed | Amazon's current standard | Frozen; deprecated |
The short version: EPUB for everything non-Kindle, AZW3 for any current Kindle, MOBI only for ancient hardware.
The Short Answer: Match the Format to Your Device
Skip the theory if you like. The decision fits in three lines.
- Modern Kindle (Paperwhite, Oasis, Scribe, basically anything from 2012 onward): use AZW3. It's Kindle-native, so typography and page layout survive intact.
- iPad, Android phone, Kobo, Nook, laptop, or some mix of all four: use EPUB. It's the open standard, and nearly every reader app outside Amazon speaks it fluently.
- A pre-KF8 Kindle that can't take a firmware update: use MOBI. It's dated, but it opens.
Everything below is the reasoning behind those three lines, plus the trade-offs nobody mentions until a book renders wrong.
Already know what you need? Convert EPUB to AZW3 runs in your browser and usually finishes in under a minute. No account, no installer. For a detailed walkthrough on converting EPUB to AZW3 while preserving your book's formatting, see our EPUB to AZW3 for Kindle guide.
EPUB: The Open Standard That Goes Almost Everywhere
EPUB started at the International Digital Publishing Forum and is now maintained by the W3C. Underneath, it's a zipped bundle of HTML, CSS, and images, which is why it behaves like a very small website.
That structure is what gives you reflowable text. Bump the font size and the words rewrap to fit the screen instead of shrinking a fixed page. On a phone, that single behavior is the entire reading experience.
Where EPUB shines:
- Opens in Apple Books, Kobo, Nook, Google Play Books, and dozens of desktop and mobile apps
- Reflows cleanly across screen sizes, orientations, and font settings
- Supports embedded fonts, real CSS styling, and proper chapter navigation
- Compresses well, so most text-only novels land far under 10 MB
Where it falls short:
- Kindle hardware won't display EPUB directly. Amazon converts one you send in, but you don't control the result.
- Heavily styled books can render slightly differently from app to app
Public libraries lend EPUB by default, so if you borrow books, this is the format you'll meet most often. The EPUB 3 specification has the technical detail if you want it.
AZW3: What Modern Kindles Actually Want
AZW3 is Amazon's KF8 format, introduced in 2011 and rolled out across the Kindle line afterward. It replaced the aging Mobipocket engine with something much closer to modern HTML and CSS.
In practice that means AZW3 handles everything MOBI never could: embedded fonts, tables, drop caps, fixed-layout pages for illustrated titles, and real control over margins and spacing.
Where AZW3 shines:
- Native on modern Kindle hardware, so nothing gets re-processed on the way in
- The best typography you'll get on an e-ink screen
- Copes with textbooks, reference works, and image-heavy books
Where it falls short:
- It's an Amazon format. Kobo, Nook, and most third-party apps ignore it.
- Kindles from before KF8 support can't read it at all
If you're building an archive rather than reading tonight, keep EPUB as your master copy and generate AZW3 on demand. Switching platforms later? Convert AZW3 to EPUB goes the other direction just as cleanly. Amazon's Send to Kindle page lists which file types your account accepts right now.
MOBI: Legacy Format, Narrow Use Case
MOBI came from Mobipocket, a company Amazon bought in 2005. It powered the first Kindles and stuck around for well over a decade.
It's simple, which is both the appeal and the ceiling. MOBI supports plain text, images, and limited HTML markup. No embedded fonts, no real tables, no fine layout control.
Amazon has been retiring it steadily. Send to Kindle stopped accepting MOBI uploads in 2022, and new titles arrive in newer formats. Even so, MOBI files exist by the million in personal archives, and old Kindle hardware reads them without complaint.
Pick MOBI when:
- Your Kindle predates KF8 and can't be updated
- Some very old reader app is the only thing that has to open the file
Skip MOBI when:
- The device handles AZW3 or EPUB, because there's no upside
- The book has tables, footnotes, or careful typography
Sitting on a folder of old MOBI files? Convert MOBI to EPUB turns them into something future readers can actually use, and our EPUB to MOBI walkthrough covers the return trip if you still need it. Going the other direction — upgrading AZW3 back to MOBI for ancient Kindles — is covered in AZW3 to MOBI. And if your archive holds FB2 or Lit files instead, FB2 vs EPUB and the Lit format conversion guide explain when those still matter.
Convert Between Formats with BookConv
Format choice only matters if switching is painless. That's the part BookConv handles.
Upload the file, pick the target format, watch the progress bar, download the result. Conversion runs server-side on a Calibre engine, so you get desktop-grade output without maintaining a desktop app or waiting on an installer.
What the free tier gives you:
- Up to 10 MB per file, which covers nearly every text-only novel
- 20 conversion requests per minute, with no account and no sign-up
- A metadata preview before you commit, so you can check the title and author were read correctly
- A live progress bar — heavy files hand off to a background worker queue instead of stalling your tab
When you need more room: every plan stops at 10 MB per file, so illustrated books and scanned PDFs need compression or splitting rather than a bigger plan.
Two things worth knowing before you upload. DRM-protected files are rejected on upload — we don't strip protection, so there's nothing to work around. And download links are temporary: converted files are deleted after a while, so save the result when it's ready instead of bookmarking the page.
The routes people take most:
- EPUB to AZW3 for a modern Kindle
- MOBI to EPUB to rescue an old archive
- PDF to EPUB when a fixed-layout file needs to reflow
- FB2 to EPUB for Russian-language archives
- Scanned PDF to EPUB with OCR when your PDF is an image
Desktop Calibre is still worth having if you manage thousands of books, edit metadata in bulk, or need to process files past 100 MB. For one or two books, a browser tab is faster than a library manager.
Key Takeaways
- Device decides — the right format is whatever your main reader opens natively. Everything else is one conversion away.
- EPUB is your master copy — open standard, reflowable, and both other formats can be generated from it.
- AZW3 wins on Kindle — better fonts, tables, and layout than MOBI, with no compatibility cost inside Amazon's ecosystem.
- MOBI is a fallback — keep it for pre-KF8 hardware, don't choose it otherwise.
- Convert on demand — one clean source plus a browser beats three half-maintained copies of every book.