What is a MOBI file?

MOBI is a binary format, meaningful only to a program that knows it. It is used for handing a finished file over.

The extension is .mobi, and the full name is Mobipocket E-book. Both matter less than what the file can hold, which is what the rest of this page is about.

Mobipocket published it in 2000.

Age is worth knowing here for one practical reason: the older a format is, the more programs have had time to learn it.

Reading it outside the vendor’s own software depends on work done by other people, usually by inspection rather than by documentation. That mostly works and occasionally does not, which is the reason to convert anything you intend to keep.

The people who made it have moved on, and support is being withdrawn rather than added. Anything still in this format is worth moving while the tools to move it are still around.

MOBI is not limited to one page, which matters when converting to something that is: one conversion produces one file holding a single page — the first one, unless the page is something the converter lets you pick.

Calibre and Kindle read it, and so do most programs of the same kind.

If a file will not open, the format is rarely the problem — it is more often that the program predates it. Converting to something older is the reliable way past that, and it is what the rest of this site is for.

No browser reads it.

That is the single most common reason to convert it: not that the format is bad, but that the place you want to show the file cannot read it.

MOBI is meant to be handed over rather than worked in. Editing one is possible and rarely pleasant; the sane approach is to change the source and export again.

The recurring complaints: support outside its home ground is uneven.

None of these is a reason to avoid the format. They are the things worth knowing before you are surprised by one, which is a different claim and a more useful one.

Mobipocket published MOBI in 2000 for reading books on Palm devices and early phones, and Amazon bought the company in 2005 to build the Kindle on it. Its design assumptions belong to that era: tiny screens, almost no processing power, and no expectation that a book would need to look like anything in particular.

So the styling is minimal by construction. It is HTML of roughly 1997 vintage — bold, italic, a handful of headings and simple images — with no proper stylesheet support, no embedded fonts, no tables worth the name and no fixed layouts. For a novel that is entirely sufficient, which is why it lasted twenty years.

Send to Kindle no longer takes MOBI. The replacement is EPUB, which Amazon converts on their side, and the change is what turned MOBI from an old format into a dead one for its main purpose.

Books already on a device keep working, and sideloading over USB still works for many older Kindles. But a MOBI is now a file to convert rather than a file to send, and anyone maintaining a library should treat it as a legacy container rather than a target.

A file can hold more than one version of the book. Later ones carry both the original MOBI content and a KF8 section — the AZW3 generation, with real CSS — so that a modern Kindle uses one and an ancient one uses the other.

That is worth knowing when converting. A converter that reads the older section produces a book stripped of styling that was present in the file all along, and the difference is very visible on anything with more than plain paragraphs. Calibre picks the better section; simpler tools do not always.

Books bought from the Kindle store generally carry digital rights management, which encrypts the content against an account. A protected MOBI cannot be opened by other software and cannot be converted — a converter reports an error rather than producing a book, and that is the encryption working rather than a fault.

Plenty of MOBI files have none: books from authors and publishers who declined to apply it, files you made yourself, material sideloaded from elsewhere. The quick test is to convert the file in Calibre, not to look at it: DRM leaves the header and the cover readable, so a protected book still shows up in the library with its title and artwork and only fails when the text is asked for. Removing it is a separate question with answers that differ by country, and not something this site does.

Calibre opens unprotected MOBI files in its own viewer and is the practical desktop answer. Amazon’s Kindle applications open anything tied to your account. Older Kindles read sideloaded files over USB.

What will not open one is Apple Books, a browser, any non-Kindle e-reader, or a phone without a specific application. For all of those the answer is conversion, and there is no reason to keep the original format in the loop.

This is the right move for anything unprotected, and the conversion is straightforward because both formats are HTML underneath. Text, chapter structure, images and the table of contents come across cleanly.

What is worth checking afterwards is metadata rather than content — author names sort oddly when the fields were filled in loosely, and series information is stored inconsistently across the format’s history. Convert one book, look at how it lands in your library, then convert the rest. EPUB is also what Send to Kindle now wants, so the conversion serves both a general library and the Kindle itself.

One lineage, in order. MOBI is the original with minimal styling. AZW is MOBI in Amazon’s wrapper. AZW3 is Kindle Format 8, with real CSS and embedded fonts. KFX is the current format that modern Kindles download, more capable and considerably more closed.

For sideloading onto a Kindle today, AZW3 or EPUB. For a library that should still be readable in ten years, EPUB — an open standard read by everything except Kindle hardware, which now accepts it anyway. MOBI belongs to neither list.