EPUB is a reflowable book split into chapters. AZW3 is the current Kindle format, which replaced MOBI. calibre sits between the two, and this page says exactly what happens to an EPUB file on the way to becoming an AZW3 one.
What runs when an EPUB file becomes AZW3
calibre the ebook-convert binary, on a machine we rent and watch. Your EPUB file is uploaded once, calibre runs once, the AZW3 comes back, and neither file is kept. EPUB to AZW3 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 EPUB into the AZW3
The reading order and the chapter split of the EPUB file. calibre rebuilds it as a book on the way to AZW3, rather than as a pile of pages.
Headings, and with them the table of contents: EPUB and AZW3 both carry a heading hierarchy calibre can map one onto the other.
Tables, as tables. EPUB and AZW3 can each hold one, so calibre does not have to flatten them.
Cover art and book metadata, which calibre carries from the EPUB container into the AZW3 one.
What EPUB to AZW3 costs you
Fixed layout. calibre reflows the EPUB text by design, because an AZW3 is meant to be read at any font size on any screen width.
Nothing we could measure, which this page used to deny. The same EPUB carrying a heading, a table, bold text, a colour and a font comes back out of the AZW3 with all five of them still in it. Whether a device renders them well is a separate question, and one no measurement here reaches.
If a model is going to read the AZW3
Structure is what survives from the EPUB file, and structure is what a model needs. EPUB in, AZW3 out, with the heading tree intact instead of flattened into one long paragraph.
EPUB to AZW3, measured rather than promised
Running EPUB to AZW3 against the real engine with a real file, calibre wrote 61,245 bytes of AZW3 in 0.6 seconds, machine otherwise idle. That is one EPUB file on one day and not an average, which is why the number is given together with the file that produced it.
The EPUB to AZW3 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 AZW3 file has anything inside it.
The same EPUB file, sent somewhere else
Other outputs the probe measured out of an EPUB file, so the cost of choosing AZW3 can be read against something. Sizes do not compare across engines, because each family was probed with its own EPUB witness file.
EPUB to PDF: 61,941 bytes in 1.9 seconds, verified by a format signature.
EPUB to MD: 457 bytes in 0.1 seconds, verified by a pandoc round trip.
EPUB to DOCX: 10,326 bytes in 0.1 seconds, verified by a pandoc round trip.
EPUB to TXT: 286 bytes in under a tenth of a second, verified by reading the text back.
EPUB to HTML: 4,104 bytes in 0.1 seconds, verified by a pandoc round trip.
EPUB to RTF: 822 bytes in under a tenth of a second, verified by a pandoc round trip.
Other ways into AZW3, and what they measured
Among the published routes into AZW3, EPUB is the third largest output of the 9 measured. The witness files differ, so this ranks the probe run and not your document.
PDF to AZW3: 58,904 bytes in 0.8 seconds.
DOCX to AZW3: 61,489 bytes in 0.8 seconds.
HTML to AZW3: 10,818 bytes in 0.6 seconds.
MD to AZW3: 10,849 bytes in 0.6 seconds.
MOBI to AZW3: 61,343 bytes in 0.6 seconds.
What we will not pretend about EPUB to AZW3
An EPUB file over 50 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
An EPUB to AZW3 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 EPUB 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 AZW3 timing below is a measurement of one small file and not a guarantee about your thousand page one.
One EPUB file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
EPUB to AZW3: what people ask
What actually converts my EPUB file to AZW3?
calibre does it, the ebook-convert binary, on a machine we rent and watch. Not a browser trick and not somebody else service: the EPUB file is uploaded once, calibre runs once, the AZW3 comes back, and neither file is kept afterwards.
How long does EPUB to AZW3 take?
On the file the probe used, calibre took 0.6 seconds and wrote 61,245 bytes of AZW3. That is one real measurement on one real EPUB file, not an average and not a promise about yours: a larger EPUB takes longer, and past 120 seconds the run is stopped.
What do I lose going from EPUB to AZW3?
The one to know about first: Fixed layout. calibre reflows the EPUB text by design, because an AZW3 is meant to be read at any font size on any screen width.
Does the table of contents survive from EPUB to AZW3?
It does, as long as your EPUB file has real headings rather than text that merely looks like headings. calibre builds the AZW3 contents from the heading structure it finds, and hand-formatted big bold lines are not a structure.
Is EPUB to AZW3 free?
There is a free allowance every month, and one EPUB to AZW3 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 AZW3.