Markdown is markup you can read with your own eyes, and the format language models read best. 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 MD file on the way to becoming a MOBI one.
What runs when an MD file becomes MOBI
calibre the ebook-convert binary, on a machine we rent and watch. Your MD file is uploaded once, calibre runs once, the MOBI comes back, and neither file is kept. MD 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 MD into the MOBI
The reading order and the chapter split of the MD 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: MD and MOBI both carry a heading hierarchy calibre can map one onto the other.
Cover art and book metadata, which calibre carries from the MD container into the MOBI one.
What MD to MOBI costs you
Fixed layout. calibre reflows the MD 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 MD was saving it. If a model is the reader, the MD you started from is already the file to hand it.
MD to MOBI, measured rather than promised
Running MD to MOBI against the real engine with a real file, calibre wrote 9,300 bytes of MOBI in 0.7 seconds, machine otherwise idle. That is one MD file on one day and not an average, which is why the number is given together with the file that produced it.
The MD 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 MD file, sent somewhere else
Other outputs the probe measured out of an MD 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 MD witness file.
MD to PDF: 25,764 bytes in 1.9 seconds, verified by a format signature.
MD to DOCX: 10,106 bytes in 0.1 seconds, verified by a pandoc round trip.
MD to EPUB: 5,034 bytes in 0.1 seconds, verified by a pandoc round trip.
MD to TXT: 264 bytes in under a tenth of a second, verified by reading the text back.
MD to HTML: 4,018 bytes in 0.1 seconds, verified by a pandoc round trip.
MD to RTF: 687 bytes in under a tenth of a second, verified by a pandoc round trip.
Other ways into MOBI, and what they measured
Among the published routes into MOBI, MD is the sixth largest output of the 9 measured. The witness files differ, so this ranks the probe run and not your document.
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.
EPUB to MOBI: 59,812 bytes in 0.7 seconds.
PDF to MOBI: 57,343 bytes in 0.9 seconds.
What we will not pretend about MD to MOBI
An MD file over 50 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
An MD 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 MD 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 MD file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
MD to MOBI: what people ask
What actually converts my MD 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 MD file is uploaded once, calibre runs once, the MOBI comes back, and neither file is kept afterwards.
How long does MD to MOBI take?
On the file the probe used, calibre took 0.7 seconds and wrote 9,300 bytes of MOBI. That is one real measurement on one real MD file, not an average and not a promise about yours: a larger MD takes longer, and past 120 seconds the run is stopped.
What do I lose going from MD to MOBI?
The one to know about first: Fixed layout. calibre reflows the MD 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 MD to MOBI?
It does, as long as your MD 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 MD to MOBI free?
There is a free allowance every month, and one MD 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.