MOBI is the old Kindle format, and a very poor carrier of formatting. TXT is plain text, with no markup of any kind. calibre sits between the two, and this page says exactly what happens to a MOBI file on the way to becoming a TXT one.
What runs when a MOBI file becomes TXT
calibre the ebook-convert binary, on a machine we rent and watch. Your MOBI file is uploaded once, calibre runs once, the TXT comes back, and neither file is kept. MOBI to TXT 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 MOBI into the TXT
The reading order and the chapter split of the MOBI file. calibre rebuilds it as a book on the way to TXT, rather than as a pile of pages.
Cover art and book metadata, which calibre carries from the MOBI container into the TXT one.
What MOBI to TXT costs you
Fixed layout. calibre reflows the MOBI text by design, because a TXT is meant to be read at any font size on any screen width.
Heading tags, tables, bold, text colour and font choices, 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 TXT does not carry heading tags, tables, bold, text colour and font choices 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 TXT
The words all survive, and the heading STRUCTURE does not: TXT has no tag for a heading, so calibre writes them as styled paragraphs. A model reading the TXT gets the text in order and has to infer the sections from it, which is exactly the work MOBI was saving it. If a model is the reader, convert to Markdown instead.
MOBI to TXT, measured rather than promised
Running MOBI to TXT against the real engine with a real file, calibre wrote 219 bytes of TXT in 0.5 seconds, machine otherwise idle. That is one MOBI file on one day and not an average, which is why the number is given together with the file that produced it.
The MOBI to TXT verdict was reached by reading the text back, meaning the output decodes as readable text and still carries the witness word, which is the only check a plain text target allows. 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 TXT file has anything inside it.
The same MOBI file, sent somewhere else
Other outputs the probe measured out of a MOBI file, so the cost of choosing TXT can be read against something. Sizes do not compare across engines, because each family was probed with its own MOBI witness file.
MOBI to PDF: 62,207 bytes in 1.9 seconds, verified by a format signature.
MOBI to DOCX: 24,070 bytes in 0.6 seconds, verified by a format signature.
MOBI to EPUB: 22,644 bytes in 0.6 seconds, verified by a format signature.
MOBI to RTF: 2,489 bytes in 0.6 seconds, verified by a format signature.
MOBI to AZW3: 61,343 bytes in 0.6 seconds, verified by a format signature.
Other ways into TXT, and what they measured
Among the published routes into TXT, MOBI is the tenth largest output of the 11 measured. The witness files differ, so this ranks the probe run and not your document.
ODT to TXT: 264 bytes in 0.1 seconds.
RTF to TXT: 263 bytes in under a tenth of a second.
DOCX to TXT: 264 bytes in 0.1 seconds.
PDF to TXT: 219 bytes in 0.8 seconds.
DOC to TXT: 268 bytes in 1.1 seconds.
What we will not pretend about MOBI to TXT
A MOBI file over 50 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
A MOBI to TXT 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. TXT 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 MOBI 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 TXT timing below is a measurement of one small file and not a guarantee about your thousand page one.
One MOBI file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
MOBI to TXT: what people ask
What actually converts my MOBI file to TXT?
calibre does it, the ebook-convert binary, on a machine we rent and watch. Not a browser trick and not somebody else service: the MOBI file is uploaded once, calibre runs once, the TXT comes back, and neither file is kept afterwards.
How long does MOBI to TXT take?
On the file the probe used, calibre took 0.5 seconds and wrote 219 bytes of TXT. That is one real measurement on one real MOBI file, not an average and not a promise about yours: a larger MOBI takes longer, and past 120 seconds the run is stopped.
What do I lose going from MOBI to TXT?
The one to know about first: Fixed layout. calibre reflows the MOBI text by design, because a TXT is meant to be read at any font size on any screen width.
Does the table of contents survive from MOBI to TXT?
It does, as long as your MOBI file has real headings rather than text that merely looks like headings. calibre builds the TXT contents from the heading structure it finds, and hand-formatted big bold lines are not a structure.
Is MOBI to TXT free?
There is a free allowance every month, and one MOBI to TXT 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 TXT.