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. MP3 is MPEG-1 Audio Layer III, lossy, and the one audio format that every device made since 1995 can play. ffmpeg sits between the two, and this page says exactly what happens to an MP4 file on the way to becoming an MP3 one.
What runs when an MP4 file becomes MP3
ffmpeg held to a single thread, on a machine we rent and watch. Your MP4 file is uploaded once, ffmpeg runs once, the MP3 comes back, and neither file is kept. MP4 to MP3 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 MP3
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 MP3 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 MP3 unchanged wherever the encoder accepts them, and resampled only where it cannot.
What MP4 to MP3 costs you
The picture, all of it, and on purpose. The MP4 video stream is dropped before any encoder sees it, and the MP3 that comes back is sound alone. Nothing in the MP3 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 MP3 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 MP3 compresses the decoded result again. Two lossy passes do not cancel each other out, they stack.
If a model is going to read the MP3
An MP3 file is a waveform, and a language model reads text, so handing an assistant this MP3 directly gets you nothing. The MP3 becomes useful the moment somebody turns the sound into words, and that is a separate job with its own page here. MP4 to MP3 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 MP3 against the real engine with a real file, ffmpeg wrote 24,780 bytes of MP3 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 MP3 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 MP3 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 MP3 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 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 M4A: 27,723 bytes in 0.2 seconds, verified by ffprobe.
MP4 to OPUS: 27,864 bytes in 0.2 seconds, verified by ffprobe.
Other ways into MP3, and what they measured
Among the published routes into MP3, MP4 is the second largest output of the 22 measured. The witness files differ, so this ranks the probe run and not your document.
MOV to MP3: 24,768 bytes in 0.1 seconds.
AVI to MP3: 24,711 bytes in 0.2 seconds.
MKV to MP3: 24,677 bytes in 0.1 seconds.
WEBM to MP3: 24,429 bytes in 0.1 seconds.
WMV to MP3: 24,259 bytes in 0.1 seconds.
What we will not pretend about MP4 to MP3
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 MP3 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 MP3 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 MP3 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 MP3.
MP4 to MP3 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 MP3 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 MP3 is a container and codec operation, and the MP3 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 MP3: what people ask
What actually converts my MP4 file to MP3?
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 MP3 comes back, and neither file is kept afterwards.
How long does MP4 to MP3 take?
The probe measured 0.2 seconds for 24,780 bytes of MP3, 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 MP3?
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 MP3 that comes back is sound alone. Nothing in the MP3 remembers there was ever an image, and no later conversion can put one back.
Does the MP3 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 MP3 free?
There is a free allowance every month, and one MP4 to MP3 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.