TXT is plain text, with no markup of any kind. HTML is the markup of the web, with native headings and tables. pandoc sits between the two, and this page says exactly what happens to a TXT file on the way to becoming an HTML one.
What runs when a TXT file becomes HTML
pandoc in two passes, on a machine we rent and watch. Your TXT file is uploaded once, pandoc runs once, the HTML comes back, and neither file is kept. TXT to HTML 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 HTML
Lists from the TXT file, nesting included, plus code blocks, block quotes and links, all re-expressed in HTML.
Images: pandoc extracts them out of the TXT file into a working directory and re-embeds them in the HTML, so nothing ends up pointing at a file that no longer exists.
What TXT to HTML costs you
The presentation layer. pandoc does not translate the TXT file into HTML, 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 HTML
Structure is what survives from the TXT file, and structure is what a model needs. TXT in, HTML out, with the heading tree intact instead of flattened into one long paragraph.
TXT to HTML, measured rather than promised
Running TXT to HTML against the real engine with a real file, pandoc wrote 3,896 bytes of HTML 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 HTML 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 HTML 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 HTML 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 DOCX: 9,900 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 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 HTML, and what they measured
Among the published routes into HTML, TXT is the seventh largest output of the 12 measured. The witness files differ, so this ranks the probe run and not your document.
DOC to HTML: 1,626 bytes in 1.1 seconds.
XLSX to HTML: 2,269 bytes in 1.1 seconds.
PPTX to HTML: 14,373 bytes in 1.5 seconds.
XLS to HTML: 2,101 bytes in 1.0 seconds.
CSV to HTML: 3,894 bytes in 0.1 seconds.
What we will not pretend about TXT to HTML
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 HTML 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 HTML 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 HTML 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 HTML: what people ask
What actually converts my TXT file to HTML?
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 HTML comes back, and neither file is kept afterwards.
How long does TXT to HTML take?
On the file the probe used, pandoc took 0.1 seconds and wrote 3,896 bytes of HTML. 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 HTML?
The one to know about first: The presentation layer. pandoc does not translate the TXT file into HTML, 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 TXT file still tables in the HTML?
Yes. pandoc reads the TXT tables into its own document tree and writes them back out in HTML syntax, so what you get is a table a machine can parse rather than a picture of one.
Is TXT to HTML free?
There is a free allowance every month, and one TXT to HTML 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 HTML.