AZW3 is the current Kindle format, which replaced MOBI. 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 an AZW3 file on the way to becoming a MOBI one.
What runs when an AZW3 file becomes MOBI
calibre the ebook-convert binary, on a machine we rent and watch. Your AZW3 file is uploaded once, calibre runs once, the MOBI comes back, and neither file is kept. AZW3 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 AZW3 into the MOBI
The reading order and the chapter split of the AZW3 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: AZW3 and MOBI both carry a heading hierarchy calibre can map one onto the other.
Cover art and book metadata, which calibre carries from the AZW3 container into the MOBI one.
What AZW3 to MOBI costs you
Fixed layout. calibre reflows the AZW3 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.
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 AZW3 was saving it. If a model is the reader, convert to Markdown instead.
AZW3 to MOBI, measured rather than promised
Running AZW3 to MOBI against the real engine with a real file, calibre wrote 59,812 bytes of MOBI in 0.8 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 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 AZW3 file, sent somewhere else
Other outputs the probe measured out of an AZW3 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 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 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 MOBI, and what they measured
Among the published routes into MOBI, AZW3 is the third largest output of the 9 measured. The witness files differ, so this ranks the probe run and not your document.
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.
TXT to MOBI: 9,184 bytes in 0.7 seconds.
What we will not pretend about AZW3 to MOBI
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 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 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 MOBI 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 MOBI: what people ask
What actually converts my AZW3 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 AZW3 file is uploaded once, calibre runs once, the MOBI comes back, and neither file is kept afterwards.
How long does AZW3 to MOBI take?
On the file the probe used, calibre took 0.8 seconds and wrote 59,812 bytes of MOBI. 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 MOBI?
The one to know about first: Fixed layout. calibre reflows the AZW3 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 AZW3 to MOBI?
It does, as long as your AZW3 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 AZW3 to MOBI free?
There is a free allowance every month, and one AZW3 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.