DOCX is Word in the OOXML format, with headings, tables and named styles. MOBI is the old Kindle format, and a very poor carrier of formatting. calibre sits between the two, and this page says exactly what happens to a DOCX file on the way to becoming a MOBI one.
What runs when a DOCX file becomes MOBI
calibre the ebook-convert binary, on a machine we rent and watch. Your DOCX file is uploaded once, calibre runs once, the MOBI comes back, and neither file is kept. DOCX to MOBI is one of the 3649 pairs that engine was probed on with a real file, which is why it has a page here and why the pairs the probe could not prove do not.
What survives from the DOCX into the MOBI
The reading order and the chapter split of the DOCX file. calibre rebuilds it as a book on the way to MOBI, rather than as a pile of pages.
Headings, and with them the table of contents: DOCX and MOBI both carry a heading hierarchy calibre can map one onto the other.
Cover art and book metadata, which calibre carries from the DOCX container into the MOBI one.
What DOCX to MOBI costs you
Fixed layout. calibre reflows the DOCX text by design, because a MOBI is meant to be read at any font size on any screen width.
Heading tags, and this is measured rather than assumed: we put an EPUB carrying a heading, a table, bold text, a colour and a font through every ebook target and read each result back with calibre. The MOBI does not carry heading tags at all. What a device then DISPLAYS is a separate question, and one no measurement here reaches.
Where DOCX and MOBI files come from
DOCX. Word writes it, and so does Google Docs on export. It is ISO/IEC 29500, a zip of XML, which is what lets a program read the document without opening Word.
If a model is going to read the MOBI
The words all survive, and the heading STRUCTURE does not: MOBI has no tag for a heading, so calibre writes them as styled paragraphs. A model reading the MOBI gets the text in order and has to infer the sections from it, which is exactly the work DOCX was saving it. If a model is the reader, convert to Markdown instead.
DOCX to MOBI, measured rather than promised
Running DOCX to MOBI against the real engine with a real file, calibre wrote 59,876 bytes of MOBI in 0.8 seconds, machine otherwise idle. That is one DOCX file on one day and not an average, which is why the number is given together with the file that produced it.
The DOCX to MOBI verdict was reached by a format signature, meaning at least four bytes unique to the format were found at the right offset. Which check was used matters, because they do not all prove the same thing, and a status code of 200 proves nothing whatsoever about whether the MOBI file has anything inside it.
The same DOCX file, sent somewhere else
Other outputs the probe measured out of a DOCX file, so the cost of choosing MOBI can be read against something. Sizes do not compare across engines, because each family was probed with its own DOCX witness file.
DOCX to PDF: 11,917 bytes in 1.2 seconds, verified by a format signature.
DOCX to MD: 266 bytes in 0.1 seconds, verified by a pandoc round trip.
DOCX to EPUB: 5,040 bytes in 0.1 seconds, verified by a pandoc round trip.
DOCX to TXT: 264 bytes in 0.1 seconds, verified by reading the text back.
DOCX to HTML: 4,018 bytes in 0.1 seconds, verified by a pandoc round trip.
DOCX to RTF: 4,205 bytes in 1.1 seconds, verified by reading the internal type and finding a witness word.
Other ways into MOBI, and what they measured
Among the published routes into MOBI, DOCX is the first largest output of the 9 measured. The witness files differ, so this ranks the probe run and not your document.
AZW3 to MOBI: 59,812 bytes in 0.8 seconds.
HTML to MOBI: 9,294 bytes in 0.6 seconds.
MD to MOBI: 9,300 bytes in 0.7 seconds.
ODT to MOBI: 9,422 bytes in 0.9 seconds.
RTF to MOBI: 9,262 bytes in 0.7 seconds.
What we will not pretend about DOCX to MOBI
A DOCX file over 50 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
A DOCX to MOBI run that passes 120 seconds is killed, and the calibre process is killed with it. A run left behind would sit on one of the machine's two cores until somebody noticed.
Your headings stop being headings. MOBI has no tag for one, so calibre writes them as styled paragraphs. The table of contents is built before that happens and survives; what you lose is the structure INSIDE the text, which matters the moment something reads the file rather than displays it.
The DOCX file used to prove this pair was short. calibre is allowed 120 seconds and a long book can genuinely need all of them, so the MOBI timing below is a measurement of one small file and not a guarantee about your thousand page one.
One DOCX file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
DOCX to MOBI: what people ask
What actually converts my DOCX file to MOBI?
calibre does it, the ebook-convert binary, on a machine we rent and watch. Not a browser trick and not somebody else service: the DOCX file is uploaded once, calibre runs once, the MOBI comes back, and neither file is kept afterwards.
How long does DOCX to MOBI take?
On the file the probe used, calibre took 0.8 seconds and wrote 59,876 bytes of MOBI. That is one real measurement on one real DOCX file, not an average and not a promise about yours: a larger DOCX takes longer, and past 120 seconds the run is stopped.
What do I lose going from DOCX to MOBI?
The one to know about first: Fixed layout. calibre reflows the DOCX text by design, because a MOBI is meant to be read at any font size on any screen width.
Does the table of contents survive from DOCX to MOBI?
It does, as long as your DOCX file has real headings rather than text that merely looks like headings. calibre builds the MOBI contents from the heading structure it finds, and hand-formatted big bold lines are not a structure.
Is DOCX to MOBI free?
There is a free allowance every month, and one DOCX to MOBI conversion costs half a credit against it. When the allowance runs out the tool says so and stops, rather than quietly handing you a worse MOBI.