TXT is plain text, with no markup of any kind. DOCX is Word in the OOXML format, with headings, tables and named styles. pandoc sits between the two, and this page says exactly what happens to a TXT file on the way to becoming a DOCX one.
What runs when a TXT file becomes DOCX
pandoc in two passes, on a machine we rent and watch. Your TXT file is uploaded once, pandoc runs once, the DOCX comes back, and neither file is kept. TXT to DOCX 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 TXT into the DOCX
Lists from the TXT file, nesting included, plus code blocks, block quotes and links, all re-expressed in DOCX.
Images: pandoc extracts them out of the TXT file into a working directory and re-embeds them in the DOCX, so nothing ends up pointing at a file that no longer exists.
What TXT to DOCX costs you
Nothing that was ever there, which is the honest answer for this pair: a TXT file has no page layout to lose. The DOCX you get uses the default Word template, so it is a plain document rather than a designed one.
Where TXT and DOCX 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 DOCX
DOCX is a good destination for a person and a mediocre one for a model: the structure is real but buried in XML with a great deal of packaging around it. If the next reader of this TXT file is an assistant rather than a colleague, TXT to Markdown is the shorter path and costs fewer tokens for exactly the same content.
TXT to DOCX, measured rather than promised
Running TXT to DOCX against the real engine with a real file, pandoc wrote 9,900 bytes of DOCX in 0.1 seconds, machine otherwise idle. That is one TXT file on one day and not an average, which is why the number is given together with the file that produced it.
The TXT to DOCX 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 DOCX file has anything inside it.
The same TXT file, sent somewhere else
Other outputs the probe measured out of a TXT file, so the cost of choosing DOCX can be read against something. Sizes do not compare across engines, because each family was probed with its own TXT witness file.
TXT to PDF: 12,976 bytes in 1.8 seconds, verified by a format signature.
TXT to MD: 210 bytes in 0.1 seconds, verified by a pandoc round trip.
TXT to EPUB: 4,955 bytes in 0.1 seconds, verified by a pandoc round trip.
TXT to HTML: 3,896 bytes in 0.1 seconds, verified by a pandoc round trip.
TXT to RTF: 484 bytes in under a tenth of a second, verified by a pandoc round trip.
TXT to ODT: 6,941 bytes in 0.1 seconds, verified by a pandoc round trip.
Other ways into DOCX, and what they measured
Among the published routes into DOCX, TXT is the ninth largest output of the 12 measured. The witness files differ, so this ranks the probe run and not your document.
DOC to DOCX: 5,462 bytes in 1.1 seconds.
TEX to DOCX: 10,172 bytes in 0.1 seconds.
AZW3 to DOCX: 23,936 bytes in 0.6 seconds.
CSV to DOCX: 9,945 bytes in 0.1 seconds.
JSON to DOCX: 10,106 bytes in 0.1 seconds.
What we will not pretend about TXT to DOCX
A TXT file over 25 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
A TXT to DOCX 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 TXT to DOCX 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 DOCX 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 TXT file goes through LibreOffice or calibre, both of which genuinely can.
One TXT file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
TXT to DOCX: what people ask
What actually converts my TXT file to DOCX?
pandoc does it, in two passes, on a machine we rent and watch. Not a browser trick and not somebody else service: the TXT file is uploaded once, pandoc runs once, the DOCX comes back, and neither file is kept afterwards.
How long does TXT to DOCX take?
On the file the probe used, pandoc took 0.1 seconds and wrote 9,900 bytes of DOCX. That is one real measurement on one real TXT file, not an average and not a promise about yours: a larger TXT takes longer, and past 60 seconds the run is stopped.
What do I lose going from TXT to DOCX?
The one to know about first: Nothing that was ever there, which is the honest answer for this pair: a TXT file has no page layout to lose. The DOCX you get uses the default Word template, so it is a plain document rather than a designed one.
Are the tables in my TXT file still tables in the DOCX?
Yes. pandoc reads the TXT tables into its own document tree and writes them back out in DOCX syntax, so what you get is a table a machine can parse rather than a picture of one.
Is TXT to DOCX free?
There is a free allowance every month, and one TXT to DOCX 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 DOCX.