MOBI is the old Kindle format, and a very poor carrier of formatting. DOCX is Word in the OOXML format, with headings, tables and named styles. calibre sits between the two, and this page says exactly what happens to a MOBI file on the way to becoming a DOCX one.
What runs when a MOBI file becomes DOCX
calibre the ebook-convert binary, on a machine we rent and watch. Your MOBI file is uploaded once, calibre runs once, the DOCX comes back, and neither file is kept. MOBI to DOCX 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 MOBI into the DOCX
The reading order and the chapter split of the MOBI file. calibre rebuilds it as a book on the way to DOCX, rather than as a pile of pages.
Headings, and with them the table of contents: MOBI and DOCX both carry a heading hierarchy calibre can map one onto the other.
Cover art and book metadata, which calibre carries from the MOBI container into the DOCX one.
What MOBI to DOCX costs you
Fixed layout. calibre reflows the MOBI text by design, because a DOCX is meant to be read at any font size on any screen width.
Where MOBI and DOCX 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 DOCX
DOCX is a good destination for a person and a mediocre one for a model: the structure is real but buried in XML with a great deal of packaging around it. If the next reader of this MOBI file is an assistant rather than a colleague, MOBI to Markdown is the shorter path and costs fewer tokens for exactly the same content.
MOBI to DOCX, measured rather than promised
Running MOBI to DOCX against the real engine with a real file, calibre wrote 24,070 bytes of DOCX in 0.6 seconds, machine otherwise idle. That is one MOBI file on one day and not an average, which is why the number is given together with the file that produced it.
The MOBI to DOCX 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 DOCX file has anything inside it.
The same MOBI file, sent somewhere else
Other outputs the probe measured out of a MOBI file, so the cost of choosing DOCX can be read against something. Sizes do not compare across engines, because each family was probed with its own MOBI witness file.
MOBI to PDF: 62,207 bytes in 1.9 seconds, verified by a format signature.
MOBI to EPUB: 22,644 bytes in 0.6 seconds, verified by a format signature.
MOBI to TXT: 219 bytes in 0.5 seconds, verified by reading the text back.
MOBI to RTF: 2,489 bytes in 0.6 seconds, verified by a format signature.
MOBI to AZW3: 61,343 bytes in 0.6 seconds, verified by a format signature.
Other ways into DOCX, and what they measured
Among the published routes into DOCX, MOBI is the first largest output of the 12 measured. The witness files differ, so this ranks the probe run and not your document.
PDF to DOCX: 23,905 bytes in 0.9 seconds.
EPUB to DOCX: 10,326 bytes in 0.1 seconds.
ODT to DOCX: 5,514 bytes in 1.1 seconds.
RTF to DOCX: 5,422 bytes in 1.1 seconds.
HTML to DOCX: 10,277 bytes in 0.1 seconds.
What we will not pretend about MOBI to DOCX
A MOBI file over 50 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
A MOBI to DOCX 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.
The MOBI 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 DOCX timing below is a measurement of one small file and not a guarantee about your thousand page one.
One MOBI file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
MOBI to DOCX: what people ask
What actually converts my MOBI file to DOCX?
calibre does it, the ebook-convert binary, on a machine we rent and watch. Not a browser trick and not somebody else service: the MOBI file is uploaded once, calibre runs once, the DOCX comes back, and neither file is kept afterwards.
How long does MOBI to DOCX take?
On the file the probe used, calibre took 0.6 seconds and wrote 24,070 bytes of DOCX. That is one real measurement on one real MOBI file, not an average and not a promise about yours: a larger MOBI takes longer, and past 120 seconds the run is stopped.
What do I lose going from MOBI to DOCX?
The one to know about first: Fixed layout. calibre reflows the MOBI text by design, because a DOCX is meant to be read at any font size on any screen width.
Does the table of contents survive from MOBI to DOCX?
It does, as long as your MOBI file has real headings rather than text that merely looks like headings. calibre builds the DOCX contents from the heading structure it finds, and hand-formatted big bold lines are not a structure.
Is MOBI to DOCX free?
There is a free allowance every month, and one MOBI to DOCX 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 DOCX.