RTF is rich text that almost every word processor ever written can open. AZW3 is the current Kindle format, which replaced MOBI. calibre sits between the two, and this page says exactly what happens to an RTF file on the way to becoming an AZW3 one.
What runs when an RTF file becomes AZW3
calibre the ebook-convert binary, on a machine we rent and watch. Your RTF file is uploaded once, calibre runs once, the AZW3 comes back, and neither file is kept. RTF to AZW3 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 RTF into the AZW3
The reading order and the chapter split of the RTF file. calibre rebuilds it as a book on the way to AZW3, rather than as a pile of pages.
Headings, and with them the table of contents: RTF and AZW3 both carry a heading hierarchy calibre can map one onto the other.
Tables, as tables. RTF and AZW3 can each hold one, so calibre does not have to flatten them.
Cover art and book metadata, which calibre carries from the RTF container into the AZW3 one.
What RTF to AZW3 costs you
Fixed layout. calibre reflows the RTF text by design, because an AZW3 is meant to be read at any font size on any screen width.
Nothing we could measure, which this page used to deny. The same EPUB carrying a heading, a table, bold text, a colour and a font comes back out of the AZW3 with all five of them still in it. Whether a device renders them well is a separate question, and one no measurement here reaches.
Where RTF and AZW3 files come from
RTF. Almost everything can write it, and that is the point: an RTF is plain text you can open in a text editor and read. It was the interchange format before there was an open standard, and it survives in exports from software too old or too small to ship anything better.
If a model is going to read the AZW3
Structure is what survives from the RTF file, and structure is what a model needs. RTF in, AZW3 out, with the heading tree intact instead of flattened into one long paragraph.
RTF to AZW3, measured rather than promised
Running RTF to AZW3 against the real engine with a real file, calibre wrote 10,837 bytes of AZW3 in 0.7 seconds, machine otherwise idle. That is one RTF file on one day and not an average, which is why the number is given together with the file that produced it.
The RTF to AZW3 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 AZW3 file has anything inside it.
The same RTF file, sent somewhere else
Other outputs the probe measured out of an RTF file, so the cost of choosing AZW3 can be read against something. Sizes do not compare across engines, because each family was probed with its own RTF witness file.
RTF to PDF: 11,917 bytes in 1.1 seconds, verified by a format signature.
RTF to MD: 265 bytes in under a tenth of a second, verified by a pandoc round trip.
RTF to DOCX: 5,422 bytes in 1.1 seconds, verified by a format signature.
RTF to EPUB: 5,033 bytes in 0.1 seconds, verified by a pandoc round trip.
RTF to TXT: 263 bytes in under a tenth of a second, verified by reading the text back.
RTF to HTML: 3,985 bytes in 0.1 seconds, verified by a pandoc round trip.
Other ways into AZW3, and what they measured
Among the published routes into AZW3, RTF is the seventh largest output of the 9 measured. The witness files differ, so this ranks the probe run and not your document.
TXT to AZW3: 10,607 bytes in 0.6 seconds.
EPUB to AZW3: 61,245 bytes in 0.6 seconds.
PDF to AZW3: 58,904 bytes in 0.8 seconds.
DOCX to AZW3: 61,489 bytes in 0.8 seconds.
HTML to AZW3: 10,818 bytes in 0.6 seconds.
What we will not pretend about RTF to AZW3
An RTF file over 50 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
An RTF to AZW3 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.
The RTF 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 AZW3 timing below is a measurement of one small file and not a guarantee about your thousand page one.
One RTF file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
RTF to AZW3: what people ask
What actually converts my RTF file to AZW3?
calibre does it, the ebook-convert binary, on a machine we rent and watch. Not a browser trick and not somebody else service: the RTF file is uploaded once, calibre runs once, the AZW3 comes back, and neither file is kept afterwards.
How long does RTF to AZW3 take?
On the file the probe used, calibre took 0.7 seconds and wrote 10,837 bytes of AZW3. That is one real measurement on one real RTF file, not an average and not a promise about yours: a larger RTF takes longer, and past 120 seconds the run is stopped.
What do I lose going from RTF to AZW3?
The one to know about first: Fixed layout. calibre reflows the RTF text by design, because an AZW3 is meant to be read at any font size on any screen width.
Does the table of contents survive from RTF to AZW3?
It does, as long as your RTF file has real headings rather than text that merely looks like headings. calibre builds the AZW3 contents from the heading structure it finds, and hand-formatted big bold lines are not a structure.
Is RTF to AZW3 free?
There is a free allowance every month, and one RTF to AZW3 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 AZW3.