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

When you upload an EPUB file to Kindle Direct Publishing, Amazon’s conversion engine transforms it into KFX, the proprietary Kindle format that powers enhanced typesetting on all modern Kindle devices and apps. Your EPUB’s structure, CSS, images, and metadata are parsed and repackaged; the content stays the same, but the container changes entirely. Understanding this epub to kindle conversion process helps you anticipate (and prevent) the formatting issues that catch many self-published authors off guard.

What File Formats Does KDP Accept?

KDP accepts EPUB, DOC, DOCX, HTML, KPF (Kindle Package Format from Kindle Create), and plain text files. Of these, EPUB is the recommended input format for reflowable ebooks because it produces the most reliable, highest-quality conversion to Kindle’s internal format. Word documents convert reasonably well but are more prone to unexpected styling issues. PDF is accepted for print books but not for ebook uploads.

As of March 2025, Amazon no longer accepts MOBI files for any ebook uploads. The MOBI format had already been deprecated for reflowable books in August 2021; the March 2025 cutoff extended that to fixed-layout ebooks as well, completing the transition away from MOBI entirely.

How Does KDP Convert Your EPUB to Kindle Format?

Amazon’s conversion pipeline takes your EPUB and repackages it into KFX, the format that Kindle devices and apps actually render. The process works roughly like this: KDP’s servers parse your EPUB’s XHTML content files, CSS stylesheets, images, fonts, and navigation documents (both the NCX and the EPUB 3 nav). It then rebuilds that content in Amazon’s proprietary format, applying its own rendering rules along the way.

The conversion is not a simple repackaging. Amazon’s engine interprets your CSS and maps it to Kindle’s own typographic system. Some CSS properties translate directly; others are ignored or overridden. For example, Kindle applies full justification to body text by default regardless of your CSS text-align setting, and it enforces its own hyphenation logic as part of enhanced typesetting.

Your table of contents is rebuilt from the EPUB’s navigation document. If your EPUB has both an NCX file (EPUB 2 legacy) and an EPUB 3 <nav> element, KDP uses the EPUB 3 nav. The logical TOC that readers access through the Kindle’s “Go to” menu comes from this navigation structure, not from any HTML table of contents page you may have included in your content. Both matter: the logical TOC for device navigation, and the HTML TOC page for readers who prefer scrolling through a clickable list. For more detail on why both types of TOC are important, see how to validate your EPUB file before uploading to a retailer.

What Is KFX and Why Did Amazon Move Away 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 significantly richer set of typographic features, including improved hyphenation, kerning, ligatures, drop caps, and better image handling. The MOBI format, originally developed by Mobipocket and acquired by Amazon in 2005, had fundamental limitations that made these features impossible to implement.

When authors talk about KPF files, they are referring to Kindle Package Format, which is essentially KFX in a different container. KPF is the output format of Kindle Create and Kindle Previewer 3. If you export a KPF file from Kindle Create and upload it to KDP, Amazon does not need to convert it further because the content is already in the KFX structure. This is one advantage of using Kindle Create for formatting: you skip the conversion step entirely.

AZW3 (also called KF8, or Kindle Format 8) was the transitional format between MOBI and KFX. Some older Kindle devices still receive AZW3 files if they cannot render KFX. Amazon’s servers determine which format to deliver based on the reader’s device and app version.

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

Enhanced typesetting is Amazon’s label for the set of advanced layout features that KFX enables. When your book supports enhanced typesetting, readers get improved character spacing, better word spacing in justified text, kerning, ligatures (such as “fi” and “fl”), and more sophisticated line-breaking algorithms that reduce awkward gaps in justified paragraphs.

For your epub to kindle conversion to support enhanced typesetting, your source EPUB must be a reflowable format (not fixed-layout), and its internal structure must be clean enough for Amazon’s engine to process without errors. Books with malformed HTML, broken CSS references, or excessively complex table layouts may fail to qualify. You can check whether your book supports enhanced typesetting by opening it in Kindle Previewer 3, which displays an “Enhanced Typesetting” label when the feature is active.

Enhanced typesetting also enables Page Flip, a feature that lets readers quickly browse through pages without losing their place. If your book does not qualify for enhanced typesetting, Page Flip will not be available to readers.

What Formatting Issues Can the Conversion Cause?

Most formatting problems that appear in the Kindle version of a book are caused by issues in the source EPUB rather than by bugs in Amazon’s conversion engine. That said, the conversion does introduce some changes that authors should be aware of.

Font handling

Kindle supports embedded fonts in OTF and TTF format, but readers must manually enable “Publisher Font” in their Kindle settings to see them. If the reader has not enabled this setting, Kindle substitutes its own default font (Bookerly on most modern devices). Even when publisher fonts are enabled, Kindle may strip embedded fonts if it detects an override conflict in your CSS, such as unclosed HTML tags or conflicting font-family declarations. If fonts are important to your book’s design, include both regular and bold weights, keep the total file size as small as possible (ideally under 2 MB, since KDP’s delivery fee on the 70% royalty option is charged per megabyte), and confirm your font license permits embedding.

CSS overrides

Kindle’s rendering engine does not honour every CSS property. Properties like float, position: absolute, and complex margin values are handled inconsistently or ignored. Body text is always fully justified unless you explicitly set text-align: left on headings and other non-body elements. Amazon strongly recommends specifying CSS alignment for all headings to prevent justified headings with awkward word spacing.

Character encoding

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

Image handling

KDP accepts JPEG, PNG, GIF, BMP, and SVG images. JPEG and PNG are recommended. KDP recommends images be at least 1,000 pixels on the longest side (with a minimum of 625 pixels on the shortest side) for adequate rendering on high-resolution Kindle screens. Amazon compresses images during conversion to keep file sizes manageable. Transparent PNG backgrounds can behave unpredictably on Kindle devices that support dark mode; if your images rely on transparency, test on both light and dark backgrounds.

How to Preview Your Kindle Book Before Publishing

Kindle Previewer 3 is the essential tool for checking your 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 in Kindle Previewer is very close to what readers will see on their devices.

When you open an EPUB in Kindle Previewer, the application converts it to KPF internally and displays the result across simulated Kindle devices: Kindle E-reader, Kindle Fire tablet, Kindle for iOS, and Kindle for Android. Check your book on at least two of these views, paying particular attention to the table of contents, chapter breaks, image placement, and any embedded fonts. If Kindle Previewer reports conversion warnings or errors, address them before uploading to KDP. For a broader guide to ebook previewing tools beyond just Kindle, see how to preview your ebook on different devices before publishing.

Frequently Asked Questions

Can you upload a KFX file directly to KDP?

No. KDP does not accept KFX files as uploads. You can upload EPUB, DOC, DOCX, HTML, or KPF files. KPF (from Kindle Create or Kindle Previewer) is the closest to KFX and requires minimal additional conversion on Amazon’s end.

Does KDP still accept MOBI files?

No. As of March 18, 2025, Amazon no longer accepts MOBI files for either reflowable or fixed-layout ebooks. EPUB is the recommended replacement format.

Will my embedded fonts display on all Kindle devices?

Not automatically. Readers must enable “Publisher Font” in their Kindle display settings to see your embedded fonts. On devices that do not support KF8 or KFX rendering, embedded fonts may not display at all. Always design your book so it reads well in Kindle’s default font as a fallback.

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

This typically happens when your EPUB content files do not explicitly declare UTF-8 character encoding. Without that declaration, Amazon’s conversion engine may interpret the file as ISO-8859-1, which cannot represent curly quotes, em dashes, and other Unicode characters correctly. Add <meta charset="utf-8"> to each XHTML file in your EPUB.

Is EPUB 2 or EPUB 3 better for KDP uploads?

EPUB 3 is preferred. KDP fully supports EPUB 3 features including the HTML5 nav element, which it uses to build the Kindle’s logical table of contents. EPUB 2 files with an NCX still convert correctly, but EPUB 3 provides a more reliable conversion path and better support for accessibility features.