DOCX is Word in the OOXML format, with headings, tables and named styles. EPUB is a reflowable book split into chapters. pandoc sits between the two, and this page says exactly what happens to a DOCX file on the way to becoming an EPUB one.
What runs when a DOCX file becomes EPUB
pandoc in two passes, on a machine we rent and watch. Your DOCX file is uploaded once, pandoc runs once, the EPUB comes back, and neither file is kept. DOCX 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 DOCX into the EPUB
The heading hierarchy. pandoc parses the DOCX 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 a DOCX file through pandoc rather than through a printer.
Lists from the DOCX file, nesting included, plus code blocks, block quotes and links, all re-expressed in EPUB.
Images: pandoc extracts them out of the DOCX 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 DOCX to EPUB costs you
Page layout. DOCX carries some and EPUB carries none, so margins, page breaks, headers and footers have nowhere to land and are simply not written.
Where DOCX and EPUB files come from
DOCX. Word writes it, and so does Google Docs on export. It is ISO/IEC 29500, a zip of XML, which is what lets a program read the document without opening Word.
If a model is going to read the EPUB
Structure is what survives from the DOCX file, and structure is what a model needs. DOCX in, EPUB out, with the heading tree intact instead of flattened into one long paragraph.
DOCX to EPUB, measured rather than promised
Running DOCX to EPUB against the real engine with a real file, pandoc wrote 5,040 bytes of EPUB in 0.1 seconds, machine otherwise idle. That is one DOCX file on one day and not an average, which is why the number is given together with the file that produced it.
The DOCX 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 DOCX file, sent somewhere else
Other outputs the probe measured out of a DOCX 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 DOCX witness file.
DOCX to PDF: 11,917 bytes in 1.2 seconds, verified by a format signature.
DOCX to MD: 266 bytes in 0.1 seconds, verified by a pandoc round trip.
DOCX to TXT: 264 bytes in 0.1 seconds, verified by reading the text back.
DOCX to HTML: 4,018 bytes in 0.1 seconds, verified by a pandoc round trip.
DOCX to RTF: 4,205 bytes in 1.1 seconds, verified by reading the internal type and finding a witness word.
DOCX to ODT: 12,806 bytes in 1.2 seconds, verified by reading the internal type and finding a witness word.
Other ways into EPUB, and what they measured
Among the published routes into EPUB, DOCX is the fifth largest output of the 11 measured. The witness files differ, so this ranks the probe run and not your document.
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.
RTF to EPUB: 5,033 bytes in 0.1 seconds.
What we will not pretend about DOCX to EPUB
A DOCX file over 25 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
A DOCX 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 DOCX 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 a DOCX file goes through LibreOffice or calibre, both of which genuinely can.
One DOCX file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
DOCX to EPUB: what people ask
What actually converts my DOCX 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 DOCX file is uploaded once, pandoc runs once, the EPUB comes back, and neither file is kept afterwards.
How long does DOCX to EPUB take?
On the file the probe used, pandoc took 0.1 seconds and wrote 5,040 bytes of EPUB. That is one real measurement on one real DOCX file, not an average and not a promise about yours: a larger DOCX takes longer, and past 60 seconds the run is stopped.
What do I lose going from DOCX to EPUB?
The one to know about first: Page layout. DOCX carries some and EPUB carries none, so margins, page breaks, headers and footers have nowhere to land and are simply not written.
Are the tables in my DOCX file still tables in the EPUB?
Yes. pandoc reads the DOCX 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 DOCX to EPUB free?
There is a free allowance every month, and one DOCX 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.