AZW3 is the current Kindle format, which replaced MOBI. 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 an AZW3 file on the way to becoming a DOCX one.
What runs when an AZW3 file becomes DOCX
calibre the ebook-convert binary, on a machine we rent and watch. Your AZW3 file is uploaded once, calibre runs once, the DOCX comes back, and neither file is kept. AZW3 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 AZW3 into the DOCX
The reading order and the chapter split of the AZW3 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: AZW3 and DOCX both carry a heading hierarchy calibre can map one onto the other.
Tables, as tables. AZW3 and DOCX can each hold one, so calibre does not have to flatten them.
Cover art and book metadata, which calibre carries from the AZW3 container into the DOCX one.
What AZW3 to DOCX costs you
Fixed layout. calibre reflows the AZW3 text by design, because a DOCX is meant to be read at any font size on any screen width.
Where AZW3 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 AZW3 file is an assistant rather than a colleague, AZW3 to Markdown is the shorter path and costs fewer tokens for exactly the same content.
AZW3 to DOCX, measured rather than promised
Running AZW3 to DOCX against the real engine with a real file, calibre wrote 23,936 bytes of DOCX in 0.6 seconds, machine otherwise idle. That is one AZW3 file on one day and not an average, which is why the number is given together with the file that produced it.
The AZW3 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 AZW3 file, sent somewhere else
Other outputs the probe measured out of an AZW3 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 AZW3 witness file.
AZW3 to PDF: 62,065 bytes in 1.8 seconds, verified by a format signature.
AZW3 to EPUB: 22,666 bytes in 0.7 seconds, verified by a format signature.
AZW3 to TXT: 219 bytes in 0.6 seconds, verified by reading the text back.
AZW3 to RTF: 2,512 bytes in 0.6 seconds, verified by a format signature.
Other ways into DOCX, and what they measured
Among the published routes into DOCX, AZW3 is the second largest output of the 12 measured. The witness files differ, so this ranks the probe run and not your document.
CSV to DOCX: 9,945 bytes in 0.1 seconds.
JSON to DOCX: 10,106 bytes in 0.1 seconds.
MOBI to DOCX: 24,070 bytes in 0.6 seconds.
PDF to DOCX: 23,905 bytes in 0.9 seconds.
EPUB to DOCX: 10,326 bytes in 0.1 seconds.
What we will not pretend about AZW3 to DOCX
An AZW3 file over 50 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
An AZW3 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 AZW3 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 AZW3 file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
AZW3 to DOCX: what people ask
What actually converts my AZW3 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 AZW3 file is uploaded once, calibre runs once, the DOCX comes back, and neither file is kept afterwards.
How long does AZW3 to DOCX take?
On the file the probe used, calibre took 0.6 seconds and wrote 23,936 bytes of DOCX. That is one real measurement on one real AZW3 file, not an average and not a promise about yours: a larger AZW3 takes longer, and past 120 seconds the run is stopped.
What do I lose going from AZW3 to DOCX?
The one to know about first: Fixed layout. calibre reflows the AZW3 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 AZW3 to DOCX?
It does, as long as your AZW3 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 AZW3 to DOCX free?
There is a free allowance every month, and one AZW3 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.