Markdown is markup you can read with your own eyes, and the format language models read best. EPUB is a reflowable book split into chapters. pandoc sits between the two, and this page says exactly what happens to an MD file on the way to becoming an EPUB one.
What runs when an MD file becomes EPUB
pandoc in two passes, on a machine we rent and watch. Your MD file is uploaded once, pandoc runs once, the EPUB comes back, and neither file is kept. MD 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 MD into the EPUB
The heading hierarchy. pandoc parses the MD file into its own document tree and writes that tree out as EPUB, so a level two heading is still a level two heading and not merely bold text.
Tables, rebuilt in EPUB syntax rather than screenshotted. That is the difference between a table a machine can read and a picture of one, and it is the whole reason to send an MD file through pandoc rather than through a printer.
Lists from the MD file, nesting included, plus code blocks, block quotes and links, all re-expressed in EPUB.
Images: pandoc extracts them out of the MD file into a working directory and re-embeds them in the EPUB, so nothing ends up pointing at a file that no longer exists.
What MD to EPUB costs you
The presentation layer. pandoc does not translate the MD file into EPUB, it rebuilds it from its own document tree, so custom styles, page setup, headers and footers do not cross over. The structure does, and that is what the tree is made of.
If a model is going to read the EPUB
Structure is what survives from the MD file, and structure is what a model needs. MD in, EPUB out, with the heading tree intact instead of flattened into one long paragraph.
MD to EPUB, measured rather than promised
Running MD to EPUB against the real engine with a real file, pandoc wrote 5,034 bytes of EPUB in 0.1 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 EPUB verdict was reached by a pandoc round trip, meaning the output was read back by pandoc and had to still contain a witness word planted in the source, which proves the content travelled and not merely the container. 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 MD file, sent somewhere else
Other outputs the probe measured out of an MD 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 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 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.
MD to ODT: 7,537 bytes in 0.1 seconds, verified by a pandoc round trip.
Other ways into EPUB, and what they measured
Among the published routes into EPUB, MD is the seventh largest output of the 11 measured. The witness files differ, so this ranks the probe run and not your document.
DOCX to EPUB: 5,040 bytes in 0.1 seconds.
CSV to EPUB: 4,913 bytes in 0.1 seconds.
HTML to EPUB: 5,027 bytes in 0.1 seconds.
JSON to EPUB: 5,035 bytes in 0.1 seconds.
ODT to EPUB: 5,063 bytes in 0.1 seconds.
What we will not pretend about MD to EPUB
An MD file over 25 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
An MD to EPUB run that passes 60 seconds is killed, and the pandoc process is killed with it. A run left behind would sit on one of the machine's two cores until somebody noticed.
32 of the 935 pairs probed in the family that serves MD to EPUB failed, and this pair is not one of them. They fail for reasons worth knowing: a writer that cannot carry what the document is made of, or output the engine could not read back. They are counted here rather than hidden, because a pair that fails quietly is worse than one that fails loudly.
pandoc never writes the EPUB through a TeX engine here, and it never writes PDF at all: the eleven engines it would need are not on this machine and a TeX distribution weighs several gigabytes. Anyone who wants a PDF out of an MD file goes through LibreOffice or calibre, both of which genuinely can.
One MD file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
MD to EPUB: what people ask
What actually converts my MD file to EPUB?
pandoc does it, in two passes, on a machine we rent and watch. Not a browser trick and not somebody else service: the MD file is uploaded once, pandoc runs once, the EPUB comes back, and neither file is kept afterwards.
How long does MD to EPUB take?
On the file the probe used, pandoc took 0.1 seconds and wrote 5,034 bytes of EPUB. 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 60 seconds the run is stopped.
What do I lose going from MD to EPUB?
The one to know about first: The presentation layer. pandoc does not translate the MD file into EPUB, it rebuilds it from its own document tree, so custom styles, page setup, headers and footers do not cross over. The structure does, and that is what the tree is made of.
Are the tables in my MD file still tables in the EPUB?
Yes. pandoc reads the MD tables into its own document tree and writes them back out in EPUB syntax, so what you get is a table a machine can parse rather than a picture of one.
Is MD to EPUB free?
There is a free allowance every month, and one MD 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.