.mobi File Extension

What kind of file is a .mobi?

A .mobi file is usually a Mobipocket ebook. It commonly uses the de facto application/x-mobipocket-ebook MIME type, which is widely recognized but not registered by IANA. The proprietary binary package can contain reflowable book text derived from HTML, images, navigation, and publication metadata.¹

Classic files place the ASCII identifier BOOKMOBI, or sometimes the older TEXTREAD, at byte offset 60. Checking this signature is more reliable than trusting the suffix alone. A combined MOBI can also contain a newer Kindle Format 8 representation after its older compatibility content.¹

How do I open a .mobi on my computer?

• On Windows, macOS, and Linux, calibre can view and manage unprotected MOBI books. It also supports MOBI as an input for conversion.²
• On iOS, Android, macOS, Windows, and Linux, FBReader opens .mobi books that do not use Mobipocket DRM.³
• On an older Kindle or Mobipocket-compatible device, support depends on the device generation and the particular content carried in the file.

Web browsers do not normally include a native MOBI reader. DRM-protected books require an authorized compatible system. calibre and FBReader do not open Mobipocket DRM, and changing the filename or MIME type cannot remove that protection.³⁴

Poly support for .mobi

Poly gives successfully processed MOBI ebooks a thumbnail, paginated in-app preview, full-text search, OCR when needed, page-level visual search, and agent reading. This makes an unprotected book searchable by both its filename and readable content.

The preview uses stable pages rather than a native reflowable ebook interface, so reader themes, adjustable fonts, and MOBI-specific navigation are not interactive. DRM-protected, damaged, or unsupported books remain safely stored and searchable by filename, but their contents cannot be previewed, indexed, or read by the agent.

History of .mobi

Mobipocket SA introduced its ebook format in 2000 for Palm and Windows CE readers. The earliest books used .prc, the extension of the underlying Palm resource container. The .mobi suffix appeared later and became closely associated with Amazon after it acquired Mobipocket in 2005 and launched Kindle in 2007.¹

Amazon later moved its publishing workflow away from MOBI. Reflowable MOBI submissions ended in 2021, and fixed-layout MOBI submissions ended on March 18, 2025. Amazon now recommends EPUB, DOCX, or KPF for new Kindle publications.⁵

Working with .mobi and alternatives

For most unprotected MOBI libraries, EPUB is the practical migration format. calibre can convert MOBI to EPUB or PDF, but conversion may change the table of contents, fonts, images, footnotes, and chapter breaks.² Check the result and retain the original. If a combined MOBI carries both old and newer representations, tools may not choose or reproduce them identically.

Keep MOBI when a working legacy device specifically requires it. Prefer EPUB for a new, portable reading copy and PDF when exact pagination matters. Do not attempt to bypass DRM; use the provider's authorized app or request an accessible replacement.