What Happens to Your EPUB When Amazon Converts It to Kindle?

Reading, not publishing? If you just want an EPUB on your own Kindle, no conversion is needed: Amazon’s free Send to Kindle service accepts EPUB files directly and delivers them to your device or app. The rest of this article is for the publishing side: what happens when you upload an EPUB to Kindle Direct Publishing, and how to stop that conversion springing surprises on your finished ebook.

When you upload an EPUB to Kindle Direct Publishing, Amazon’s conversion engine rebuilds it as KFX, the Kindle format that powers Enhanced Typesetting on modern Kindle devices and apps. Your structure, CSS, images and navigation are parsed and repackaged: the content stays the same, but the container changes entirely. Knowing what the EPUB to Kindle conversion keeps, overrides and discards is how you avoid the formatting surprises that catch many self-published authors out.

What File Formats Does KDP Accept?

KDP’s supported formats page puts three formats forward as the recommended and most commonly used: Microsoft Word (DOC/DOCX), Kindle Package Format (KPF) from Kindle Create, and EPUB. It then lists further accepted formats “including but not limited to” HTML (as ZIP, HTM or HTML), Rich Text Format, plain text, and PDF.

EPUB is the format to aim for if you want the most reliable result. Word files convert acceptably but carry every quirk of the original document into the engine, which is why fonts chosen in Word so often fail to survive the trip. PDF is worth a note of its own: it is an accepted ebook upload format in those nine languages, but a PDF is a fixed page rather than reflowable text, so a reader cannot resize it comfortably on a small screen. Our comparison of EPUB and PDF covers when each one is the right choice.

Amazon confirms in its MOBI support FAQ that it stopped accepting MOBI files for fixed-layout ebooks on 18 March 2025, completing a transition that began when it ended MOBI support for reflowable books on 1 August 2021. Previously published MOBI-based ebooks do not need re-uploading unless you are updating them.

How Does KDP Convert Your EPUB to Kindle Format?

Amazon’s pipeline takes your EPUB apart and reassembles it in KFX, the format Kindle devices and apps actually render. Its servers parse the XHTML content files, the CSS, the images, the fonts and the navigation documents, then rebuild that content in Amazon’s own format while applying Amazon’s rendering rules.

The conversion is not a simple repackaging. Amazon’s engine interprets your CSS and maps it onto Kindle’s typographic system. Some properties translate directly; others are overridden or ignored. The clearest example is justification: Amazon’s Text Guidelines for reflowable books state that text in reflowable Kindle books is fully justified by default, and for that reason strongly recommend setting an explicit CSS alignment on every heading, so that headings do not inherit justification and open ugly gaps between words.

Your table of contents is rebuilt from the EPUB’s navigation structure rather than from any visible contents page. If your file carries both an NCX (the EPUB 2 method) and an EPUB 3 <nav> element, the logical TOC that fills the Kindle “Go to” menu comes from that navigation layer. The visible contents page is separate, and readers use it differently. Both matter, and our guide to the two types of ebook table of contents explains how each one is built.

The practical consequence is worth stating plainly: the engine can only work with what you give it. It fixes nothing. A chapter your source file never marked as a heading does not become one on the way through, which is why validating the EPUB before upload catches more problems than checking the Kindle version afterwards.

What Is KFX, and Why Did Amazon Move On From MOBI?

KFX (Kindle Format X) is Amazon’s current delivery format for Kindle ebooks. It replaced the older MOBI and AZW3 formats because it supports a far richer set of typographic features: improved hyphenation, kerning, ligatures, drop caps and better image handling. MOBI, developed by Mobipocket and acquired by Amazon in 2005, had structural limits that made those features impossible. Amazon’s own explanation for retiring it was that doing so simplified creating ebooks natively with Enhanced Typesetting and removed the overhead of maintaining MOBI files.

KPF (Kindle Package Format) is essentially KFX in a different container. It is what Kindle Create and Kindle Previewer 3 produce. Upload a KPF and Amazon has little left to convert, because the content is already in Kindle’s structure. That is one reason Kindle Create appeals to authors who would rather not think about conversion at all, though it comes at the cost of the design control a hand-built EPUB gives you.

AZW3 (also called KF8, or Kindle Format 8) was the transitional format between MOBI and KFX. Amazon’s servers decide which format to deliver based on the reader’s device and app version, so an older device may still receive AZW3. This is worth remembering when you test: the file a 2013 Kindle receives is not necessarily the file a 2026 Kindle receives.

What Is Enhanced Typesetting and How Does It Affect Your Book?

Enhanced Typesetting is Amazon’s label for the layout features KFX enables, and Amazon applies it only to reflowable books uploaded as EPUB, DOC, DOCX or HTML. Its Enhanced Typesetting page lists what a qualifying book gains, and the list is longer than most authors realise:

• Page Flip: readers can skim through the book without losing their place.
• Progressive download: reading can start before the download finishes.
• High definition visuals: better zooming, pinching and panning on images.
• Font size layout improvement: at large text sizes Kindle falls back to left-aligned text, or reduces a two-column landscape layout to one column, rather than leaving gaping word spaces.
• Ragged right justification: readers can switch away from the full-justified default to left-aligned text.
• Drop caps, kerning, ligatures and hyphenation: the typographic detail that separates a typeset page from a word-processed one.
• Dynamic colour contrast: text of any colour is adjusted to stay legible against the reader’s chosen theme or a page background.
• Enhanced table formatting and MathML: tables open in a pannable viewer with selectable text, and mathematical notation is supported.

Two of these change how you should think about your own styling. Because readers can turn on ragged-right themselves, forcing left alignment on body text in your CSS is unnecessary and counterproductive. And because Kindle adjusts font colours for contrast, hard-coded colours are less of a rescue than they look.

To qualify, the source file must be reflowable rather than fixed-layout, and clean enough for Amazon’s engine to process without errors. Malformed HTML, broken CSS references and very complex table layouts can all disqualify a book. You can check whether yours qualifies by opening it in Kindle Previewer 3, which displays an “Enhanced Typesetting” label when the feature is active.

What Formatting Issues Can the Conversion Cause?

Most problems that show up in the Kindle version originate in the source EPUB rather than in Amazon’s engine. The conversion does make changes of its own, though, and these are the ones that surprise people.

Body text is meant to be plain

Amazon’s Text Guidelines are unusually direct here: body text in a reflowable Kindle book “must be all defaults”. Creative styling belongs on headings, special paragraphs, footnotes and the contents page, not on the running text, because styling the body overrides the reader’s own preferred settings. In practice that means no font-size or line-height on body paragraphs, no book-wide bold or italic, and no imposed font colour throughout. Where you do colour text, Amazon recommends greys in the hex range #666 to #999 so that the result works on both white and black backgrounds.

Font handling

Kindle supports embedded OTF and TTF fonts, but readers have to enable the publisher font in their display settings to see them, and many never do. Kindle may also drop embedded fonts where it detects an override conflict, such as unclosed tags or contradictory font-family declarations. If fonts matter to your design, embed regular and bold weights, keep the file size down (on the 70% royalty option KDP deducts a delivery cost that varies with file size), and confirm the licence permits embedding rather than merely installing.

Character encoding

If your EPUB does not explicitly declare UTF-8 encoding in its content files, the conversion may fall back to ISO-8859-1 and turn smart quotes, dashes and accented characters into garbled text (the familiar “’” problem). Always declare <meta charset="utf-8"> in every XHTML file. This is one of the most common causes of ebook conversion problems.

Image handling

Amazon’s Image Guidelines for reflowable books set expectations in terms of how much of the screen an image fills rather than in raw pixels: pictorial images should occupy at least 60% of the screen width on small devices, and images carrying text, such as charts and maps, at least 80%. Cover images should be at least 1,200 pixels on their width or height. Four further rules catch people out:

• TIFF files, multi-frame GIFs and images with transparent areas are not supported. Transparency is converted to white, which is why a logo that looked fine on your screen can sit in a white box on a dark-mode e-reader.
• Use sRGB. Kindle does not support CMYK and converts it for you, with whatever colour shift that brings.
• Every image needs alt text. Decorative images should carry alt="" or role="presentation" so assistive technology skips them.
• Do not render text as an image. If the text can be separated from the artwork, it should be HTML, and captions belong in a figcaption rather than baked into the picture.

Amazon also flags blurry images and illegible text in images with a content quality warning on the book’s detail page, which is public and visible to shoppers.

How Do You Preview Your Kindle Book Before Publishing?

Kindle Previewer 3 is the tool for checking the EPUB to Kindle conversion before you publish. It is a free desktop application for Mac and Windows that uses the same rendering engine as KDP’s conversion pipeline, so what you see is close to what readers get.

When you open an EPUB in Kindle Previewer, the application converts it to KPF internally and shows the result across simulated devices: Kindle E Ink readers, Fire tablets, and the Kindle apps for iOS and Android. Check at least two of those views, and look specifically at the table of contents, chapter breaks, image placement and any embedded fonts. If Previewer reports conversion warnings or errors, deal with them before uploading rather than after.

One habit is worth building. Check the book at a large font size as well as the default, because that is where justification, tables and inline images fail first, and it is the setting a substantial share of readers actually use. For a broader look at testing beyond Kindle, see our guide to previewing your ebook on different devices.

Frequently Asked Questions

Can you upload a KFX file directly to KDP?

No. KDP does not accept KFX as an upload format. You can upload EPUB, DOC, DOCX, KPF, HTML, RTF, plain text, or (in nine languages) PDF. KPF, produced by Kindle Create or Kindle Previewer, is the closest to KFX and needs the least further conversion at Amazon’s end.

Does KDP still accept MOBI files?

No. Amazon stopped accepting MOBI files for fixed-layout ebooks on 18 March 2025, having ended MOBI support for reflowable books on 1 August 2021. EPUB, DOCX and KPF are the replacements. Previously published books do not need re-uploading unless you are updating them.

Will my embedded fonts display on all Kindle devices?

Not automatically. Readers have to enable the publisher font in their Kindle display settings, and most leave the default in place. Kindle may also drop an embedded font where it finds a conflicting CSS declaration. Design the book so that it reads well in Kindle’s default font, and treat your embedded font as an enhancement rather than the plan.

Why do my smart quotes appear as garbled characters on Kindle?

This usually happens when your EPUB’s content files do not declare UTF-8 encoding. Without that declaration the conversion may read the file as ISO-8859-1, which cannot represent curly quotes, dashes and other Unicode characters. Add a meta charset declaration of utf-8 to each XHTML file in the EPUB and re-upload.

Is EPUB 2 or EPUB 3 better for KDP uploads?

Amazon has no mandatory requirement either way and states that it accepts both EPUB 2.0 and EPUB 3.0 files that meet the Kindle Publishing Guidelines. EPUB 3 is the more useful choice for reasons that sit outside Amazon: it is what IngramSpark requires, it carries accessibility metadata properly, and its navigation document is ordinary markup you can inspect. If your tool already outputs EPUB 3, there is no reason to go back.

Why does my Kindle book look different from my EPUB?

Because Amazon applies its own rendering rules on top of your file. Body text is fully justified by default, headings inherit that justification unless you set an alignment, transparent image backgrounds become white, CMYK images are converted to sRGB, and your embedded fonts stay hidden until a reader turns publisher fonts on. None of these is a fault in your EPUB; they are the conversion doing what it is documented to do.

Does my ebook’s file size affect my royalties?

It can affect royalties. On the 70% royalty option, Amazon deducts a delivery cost that varies with the book’s file size: it publishes an average of $0.06 per unit sold, and quotes rates per megabyte by territory (€0.10 per MB in Spain, for example). The 35% option carries no delivery cost. Large uncompressed images are almost always what pushes the figure up, so sizing images to what a screen can actually show, rather than shipping print-resolution files, is the most effective lever you have.