MP4 is the MPEG-4 container, which is what a phone records, what a browser plays, and what almost every player opens without being asked twice. M4A is an MPEG-4 container holding AAC audio, which is what a voice memo and a music store purchase both are. ffmpeg sits between the two, and this page says exactly what happens to an MP4 file on the way to becoming an M4A one.
What runs when an MP4 file becomes M4A
ffmpeg held to a single thread, on a machine we rent and watch. Your MP4 file is uploaded once, ffmpeg runs once, the M4A comes back, and neither file is kept. MP4 to M4A 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 MP4 into the M4A
The sound, and nothing but the sound. ffmpeg passes -vn on every audio target, so the MP4 video stream is dropped at the demultiplexer and never reaches an encoder at all. That is why this pair is quick: the picture is the expensive part of an MP4 file, and this pass does not look at it.
The running time. The M4A plays for exactly as long as the MP4 did, because nothing on this path trims, cuts or speeds anything up.
The sample rate and the channel layout of the MP4 audio track, carried into the M4A unchanged wherever the encoder accepts them, and resampled only where it cannot.
What MP4 to M4A costs you
The picture, all of it, and on purpose. The MP4 video stream is dropped before any encoder sees it, and the M4A that comes back is sound alone. Nothing in the M4A remembers there was ever an image, and no later conversion can put one back.
Cover art carried inside the MP4. ffmpeg counts an attached picture as a video stream, so the same -vn that drops the film drops the artwork with it. Keeping it is worse: on several audio containers the mux then fails outright with an error nobody can read.
Any second audio track, and the subtitles. ffmpeg selects one stream out of the MP4, and an M4A file has nowhere to put a subtitle track in any case.
Exactness, one more time. The audio inside the MP4 was compressed once already, and M4A compresses the decoded result again. Two lossy passes do not cancel each other out, they stack.
If a model is going to read the M4A
An M4A file is a waveform, and a language model reads text, so handing an assistant this M4A directly gets you nothing. The M4A becomes useful the moment somebody turns the sound into words, and that is a separate job with its own page here. MP4 to M4A is the step before it: get the sound out of the MP4 in a container the transcriber accepts, then hand the model a transcript instead of a file it cannot open.
Running MP4 to M4A against the real engine with a real file, ffmpeg wrote 27,723 bytes of M4A in 0.2 seconds, machine otherwise idle. That is one MP4 file on one day and not an average, which is why the number is given together with the file that produced it.
The MP4 to M4A verdict was reached by ffprobe, meaning the container was actually demultiplexed, recognised under the right name, and carried at least one stream. 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 M4A file has anything inside it.
The same MP4 file, sent somewhere else
Other outputs the probe measured out of an MP4 file, so the cost of choosing M4A can be read against something. Sizes do not compare across engines, because each family was probed with its own MP4 witness file.
MP4 to GIF: 164,004 bytes in 0.2 seconds, verified by a format signature.
MP4 to MP3: 24,780 bytes in 0.2 seconds, verified by ffprobe.
MP4 to WAV: 266,318 bytes in 0.1 seconds, verified by ffprobe.
MP4 to FLAC: 156,949 bytes in 0.1 seconds, verified by ffprobe.
MP4 to OGG: 9,186 bytes in 0.1 seconds, verified by ffprobe.
MP4 to OPUS: 27,864 bytes in 0.2 seconds, verified by ffprobe.
Other ways into M4A, and what they measured
Among the published routes into M4A, MP4 is the tenth largest output of the 16 measured. The witness files differ, so this ranks the probe run and not your document.
MP3 to M4A: 27,630 bytes in 0.2 seconds.
3GP to M4A: 27,723 bytes in 0.2 seconds.
AAC to M4A: 27,924 bytes in 0.2 seconds.
AVI to M4A: 27,896 bytes in 0.2 seconds.
FLAC to M4A: 27,633 bytes in 0.2 seconds.
What we will not pretend about MP4 to M4A
An MP4 file over 150 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
An MP4 to M4A run that passes 240 seconds is killed, and the ffmpeg process is killed with it. A run left behind would sit on one of the machine's two cores until somebody noticed.
The MP4 file behind the measurement on this page was 3 seconds of 320 by 240 test pattern at 25 frames a second, with a sine wave on the audio track, built by ffmpeg for the probe itself. That is why MP4 to M4A came out at 0.2 seconds, and it is not what your file will do. The measurement proves the pair works and says almost nothing about how long yours will take.
MP4 to M4A costs half a credit per conversion, whatever the length of the MP4. That is the same as an image or a document here, and it is a measurement rather than a gesture: an audio target on this engine is done in under two seconds, where a video target can take minutes.
An MP4 longer than 60 minutes is refused before encoding. ffprobe measures the MP4 first and answers with a refusal, rather than starting a job the 240 second guard would kill halfway to the M4A.
MP4 to M4A waits at the audio door, which is 2 slots wide and gives up after 5 seconds rather than making you wait twenty times the length of the work. Until the 30th there was one door for everything, and a single video transcode starved every audio conversion for its whole duration: an M4A that needed less than a second was answered with a refusal after twenty.
Nothing here reads what is inside the MP4. No transcription, no scene detection, no summary: MP4 to M4A is a container and codec operation, and the M4A says exactly what the MP4 said, in a different wrapper. The sound is the whole payload.
One MP4 file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
MP4 to M4A: what people ask
What actually converts my MP4 file to M4A?
ffmpeg does it, held to a single thread, on a machine we rent and watch. Not a browser trick and not somebody else service: the MP4 file is uploaded once, ffmpeg runs once, the M4A comes back, and neither file is kept afterwards.
How long does MP4 to M4A take?
The probe measured 0.2 seconds for 27,723 bytes of M4A, on a 3 second file. An audio target scales far better than a video one: three minutes of source through this engine takes under a second, because only the sound is encoded. Past 240 seconds any run is stopped, and an audio one does not get close.
What do I lose going from MP4 to M4A?
The one to know about first: The picture, all of it, and on purpose. The MP4 video stream is dropped before any encoder sees it, and the M4A that comes back is sound alone. Nothing in the M4A remembers there was ever an image, and no later conversion can put one back.
Does the M4A still have the video in it?
No, and that is the entire point of the pair. The MP4 picture is dropped before any encoder touches it, so what you download is sound only. If you wanted to keep the picture you wanted a video target, and it is priced differently because it costs differently.
Is MP4 to M4A free?
There is a free allowance every month, and one MP4 to M4A conversion costs half a credit against it however long the MP4 runs. An audio target is charged per conversion and not per minute, because the engine encodes three minutes of sound in well under a second.