TXT is plain text, with no markup of any kind. 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 a TXT file on the way to becoming a MOBI one.
What runs when a TXT file becomes MOBI
calibre the ebook-convert binary, on a machine we rent and watch. Your TXT file is uploaded once, calibre runs once, the MOBI comes back, and neither file is kept. TXT 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 TXT into the MOBI
The reading order and the chapter split of the TXT file. calibre rebuilds it as a book on the way to MOBI, rather than as a pile of pages.
Cover art and book metadata, which calibre carries from the TXT container into the MOBI one.
What TXT to MOBI costs you
Fixed layout. calibre reflows the TXT 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 TXT was saving it. If a model is the reader, convert to Markdown instead.
TXT to MOBI, measured rather than promised
Running TXT to MOBI against the real engine with a real file, calibre wrote 9,184 bytes of MOBI in 0.7 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 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 TXT file, sent somewhere else
Other outputs the probe measured out of a TXT 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 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 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.
Other ways into MOBI, and what they measured
Among the published routes into MOBI, TXT is the ninth largest output of the 9 measured. The witness files differ, so this ranks the probe run and not your document.
EPUB to MOBI: 59,812 bytes in 0.7 seconds.
PDF to MOBI: 57,343 bytes in 0.9 seconds.
DOCX to MOBI: 59,876 bytes in 0.8 seconds.
AZW3 to MOBI: 59,812 bytes in 0.8 seconds.
HTML to MOBI: 9,294 bytes in 0.6 seconds.
What we will not pretend about TXT to MOBI
A TXT file over 50 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
A TXT 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 TXT 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 TXT file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
TXT to MOBI: what people ask
What actually converts my TXT 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 TXT file is uploaded once, calibre runs once, the MOBI comes back, and neither file is kept afterwards.
How long does TXT to MOBI take?
On the file the probe used, calibre took 0.7 seconds and wrote 9,184 bytes of MOBI. 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 120 seconds the run is stopped.
What do I lose going from TXT to MOBI?
The one to know about first: Fixed layout. calibre reflows the TXT 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 TXT to MOBI?
It does, as long as your TXT 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 TXT to MOBI free?
There is a free allowance every month, and one TXT 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.