MOBI is the old Kindle format, and a very poor carrier of formatting. EPUB is a reflowable book split into chapters. calibre sits between the two, and this page says exactly what happens to a MOBI file on the way to becoming an EPUB one.
What runs when a MOBI file becomes EPUB
calibre the ebook-convert binary, on a machine we rent and watch. Your MOBI file is uploaded once, calibre runs once, the EPUB comes back, and neither file is kept. MOBI 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 MOBI into the EPUB
The reading order and the chapter split of the MOBI 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: MOBI and EPUB 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 EPUB one.
What MOBI to EPUB costs you
Fixed layout. calibre reflows the MOBI 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 MOBI file, and structure is what a model needs. MOBI in, EPUB out, with the heading tree intact instead of flattened into one long paragraph.
MOBI to EPUB, measured rather than promised
Running MOBI to EPUB against the real engine with a real file, calibre wrote 22,644 bytes of EPUB 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 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 MOBI file, sent somewhere else
Other outputs the probe measured out of a MOBI 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 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 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.
MOBI to AZW3: 61,343 bytes in 0.6 seconds, verified by a format signature.
Other ways into EPUB, and what they measured
Among the published routes into EPUB, MOBI is the third largest output of the 11 measured. The witness files differ, so this ranks the probe run and not your document.
AZW3 to EPUB: 22,666 bytes in 0.7 seconds.
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.
What we will not pretend about MOBI to EPUB
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 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 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 EPUB 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 EPUB: what people ask
What actually converts my MOBI 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 MOBI file is uploaded once, calibre runs once, the EPUB comes back, and neither file is kept afterwards.
How long does MOBI to EPUB take?
On the file the probe used, calibre took 0.6 seconds and wrote 22,644 bytes of EPUB. 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 EPUB?
The one to know about first: Fixed layout. calibre reflows the MOBI 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 MOBI to EPUB?
It does, as long as your MOBI 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 MOBI to EPUB free?
There is a free allowance every month, and one MOBI 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.