For most reflowable Kindle books, EPUB is the best upload format when you want control over chapter structure and navigation. KDP also accepts Word files and several other manuscript formats, and converts whatever you give it into Amazon's own delivery format on the way to the reader.
That last part is the thing that confuses people. The "Kindle file format" is really two different questions: what you are allowed to upload, and what a Kindle stores. They are not the same file, and you only ever deal with the first one.
What you can upload to KDP
Amazon's accepted list for an ebook manuscript, as published on its own help page:
| Format | Extension | Notes |
|---|---|---|
| EPUB | .epub | EPUB 2 and EPUB 3 both accepted |
| Microsoft Word | .doc, .docx | Converted to reflowable by KDP |
| Kindle Package Format | .kpf | Produced by Kindle Create; Amazon-only |
| HTML | .html, .htm, or zipped | |
| Rich Text | .rtf | |
| Plain text | .txt | |
| Only for a fixed list of languages, and a poor fit for reflowable books |
Not on the list any more: MOBI. KDP stopped accepting it for reflowable books in 2021 and for fixed-layout books in March 2025. If a guide tells you to convert your EPUB to MOBI before uploading, it was written before then.
What a Kindle actually stores
When a reader buys your book, they do not receive the EPUB or the Word file. Amazon converts the upload into its own format for delivery — historically AZW3, more recently KFX — and that is what sits on the device. These formats are Amazon's own, are not documented for authors, and are not something you can produce or need to.
So the file extension on a Kindle is a fact about Amazon's delivery pipeline, not a decision you get to make. The decision you do make is what goes in.
Which one to upload
Three of the seven are the realistic options. The other four are accepted but rarely the right starting point for a book.
EPUB when you want to decide the structure yourself. An EPUB carries its own navigation document — the list of links that becomes the reader's chapter menu — and its own metadata. What you upload is what the reader gets. You can also run it through EPUBCheck first and know it is sound before Amazon sees it. This is the format to use if you are publishing on Apple Books or Kobo as well, because they take EPUB and nothing else.
Word when you would rather Amazon decided. KDP's converter reads the document's headings and builds the navigation from them. It does a fair job on a cleanly structured file and a poor one on a file where the chapter titles were made big and bold by hand rather than styled as headings — the two look identical on screen and only one of them becomes a chapter entry. The full comparison is in Kindle: EPUB or Word?.
KPF when Amazon is the only store you will ever use. Kindle Create produces it, and it supports some layout features the other formats do not. But it cannot be uploaded anywhere else, so a book that might one day go wide is built twice.
The reflowable question underneath all this
Every format above, except PDF, produces a reflowable book: one with no fixed pages, where the reader picks the type size and the device re-breaks the text to fit. That is what a novel, a memoir or a business book should be.
The alternative is fixed layout, where every page is a picture that does not reflow. It exists for children's picture books, comics and cookbooks, and it is a different product with a different production process. Reflowable or fixed layout covers the distinction; if your book is mostly words, the answer is reflowable and this whole page applies to you.
Reflowable is also the precondition for Amazon's Enhanced Typesetting — the hyphenation, kerning and Page Flip that make a Kindle book look typeset rather than merely displayed. Only reflowable uploads are eligible, and what keeps a file eligible is covered in Kindle Enhanced Typesetting: what it is, and how a book gets it.
The short version
- Upload EPUB if you care what the chapter navigation looks like or are publishing beyond Amazon.
- Upload Word if your headings are genuinely styled as headings and you are happy for Amazon to build the navigation.
- Do not convert anything to MOBI.
- Ignore what the Kindle stores. It is not your file and never was.
To make the file: the Kindle / KDP formatter produces the EPUB and a structured DOCX from a Word manuscript; the EPUB formatter builds a validated EPUB 3 for every store; and the EPUB validator runs EPUBCheck on any file you already have.