AZW3 is the current Kindle format, which replaced MOBI. EPUB is a reflowable book split into chapters. calibre sits between the two, and this page says exactly what happens to an AZW3 file on the way to becoming an EPUB one.
What runs when an AZW3 file becomes EPUB
calibre the ebook-convert binary, on a machine we rent and watch. Your AZW3 file is uploaded once, calibre runs once, the EPUB comes back, and neither file is kept. AZW3 to EPUB 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 EPUB
The reading order and the chapter split of the AZW3 file. calibre rebuilds it as a book on the way to EPUB, rather than as a pile of pages.
Headings, and with them the table of contents: AZW3 and EPUB both carry a heading hierarchy calibre can map one onto the other.
Tables, as tables. AZW3 and EPUB 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 EPUB one.
What AZW3 to EPUB costs you
Fixed layout. calibre reflows the AZW3 text by design, because an EPUB is meant to be read at any font size on any screen width.
If a model is going to read the EPUB
Structure is what survives from the AZW3 file, and structure is what a model needs. AZW3 in, EPUB out, with the heading tree intact instead of flattened into one long paragraph.
AZW3 to EPUB, measured rather than promised
Running AZW3 to EPUB against the real engine with a real file, calibre wrote 22,666 bytes of EPUB in 0.7 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 EPUB 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 EPUB 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 EPUB 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 DOCX: 23,936 bytes in 0.6 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 EPUB, and what they measured
Among the published routes into EPUB, AZW3 is the second largest output of the 11 measured. The witness files differ, so this ranks the probe run and not your document.
PDF to EPUB: 26,909 bytes in 0.9 seconds.
MD to EPUB: 5,034 bytes in 0.1 seconds.
DOCX to EPUB: 5,040 bytes in 0.1 seconds.
CSV to EPUB: 4,913 bytes in 0.1 seconds.
HTML to EPUB: 5,027 bytes in 0.1 seconds.
What we will not pretend about AZW3 to EPUB
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 EPUB 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 EPUB 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 EPUB: what people ask
What actually converts my AZW3 file to EPUB?
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 EPUB comes back, and neither file is kept afterwards.
How long does AZW3 to EPUB take?
On the file the probe used, calibre took 0.7 seconds and wrote 22,666 bytes of EPUB. 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 EPUB?
The one to know about first: Fixed layout. calibre reflows the AZW3 text by design, because an EPUB is meant to be read at any font size on any screen width.
Does the table of contents survive from AZW3 to EPUB?
It does, as long as your AZW3 file has real headings rather than text that merely looks like headings. calibre builds the EPUB contents from the heading structure it finds, and hand-formatted big bold lines are not a structure.
Is AZW3 to EPUB free?
There is a free allowance every month, and one AZW3 to EPUB 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 EPUB.