MOBI is the old Kindle format, and a very poor carrier of formatting. AZW3 is the current Kindle format, which replaced MOBI. calibre sits between the two, and this page says exactly what happens to a MOBI file on the way to becoming an AZW3 one.
What runs when a MOBI file becomes AZW3
calibre the ebook-convert binary, on a machine we rent and watch. Your MOBI file is uploaded once, calibre runs once, the AZW3 comes back, and neither file is kept. MOBI 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 MOBI into the AZW3
The reading order and the chapter split of the MOBI 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: MOBI and AZW3 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 AZW3 one.
What MOBI to AZW3 costs you
Fixed layout. calibre reflows the MOBI 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 MOBI file, and structure is what a model needs. MOBI in, AZW3 out, with the heading tree intact instead of flattened into one long paragraph.
MOBI to AZW3, measured rather than promised
Running MOBI to AZW3 against the real engine with a real file, calibre wrote 61,343 bytes of AZW3 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 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 MOBI file, sent somewhere else
Other outputs the probe measured out of a MOBI 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 MOBI witness file.
MOBI to PDF: 62,207 bytes in 1.9 seconds, verified by a format signature.
MOBI to DOCX: 24,070 bytes in 0.6 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.
Other ways into AZW3, and what they measured
Among the published routes into AZW3, MOBI is the second largest output of the 9 measured. The witness files differ, so this ranks the probe run and not your document.
ODT to AZW3: 11,155 bytes in 0.8 seconds.
RTF to AZW3: 10,837 bytes in 0.7 seconds.
TXT to AZW3: 10,607 bytes in 0.6 seconds.
EPUB to AZW3: 61,245 bytes in 0.6 seconds.
PDF to AZW3: 58,904 bytes in 0.8 seconds.
What we will not pretend about MOBI to AZW3
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 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 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 AZW3 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 AZW3: what people ask
What actually converts my MOBI 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 MOBI file is uploaded once, calibre runs once, the AZW3 comes back, and neither file is kept afterwards.
How long does MOBI to AZW3 take?
On the file the probe used, calibre took 0.6 seconds and wrote 61,343 bytes of AZW3. 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 AZW3?
The one to know about first: Fixed layout. calibre reflows the MOBI 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 MOBI to AZW3?
It does, as long as your MOBI 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 MOBI to AZW3 free?
There is a free allowance every month, and one MOBI 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.