MP3 is MPEG-1 Audio Layer III, lossy, and the one audio format that every device made since 1995 can play. AAC is Advanced Audio Coding on its own, lossy, and the encoding that replaced MP3 nearly everywhere it mattered. ffmpeg sits between the two, and this page says exactly what happens to an MP3 file on the way to becoming an AAC one.
What runs when an MP3 file becomes AAC
ffmpeg held to a single thread, on a machine we rent and watch. Your MP3 file is uploaded once, ffmpeg runs once, the AAC comes back, and neither file is kept. MP3 to AAC 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 MP3 into the AAC
The running time and the channel layout. A stereo MP3 comes back as a stereo AAC of the same length: this is a re-encode, not an edit, and nothing here normalises, trims or boosts the MP3.
Everything the MP3 still had. ffmpeg decodes the MP3 in full before it writes a single byte of AAC, so nothing is skipped on the way in, whatever was already missing.
What MP3 to AAC costs you
Generation loss, and it is the whole story of this pair. MP3 threw information away to be small, AAC throws more away on top of what is left, and neither pass can restore what the other removed. Converting the AAC back to MP3 afterwards makes it worse a third time, not better.
Album art, where the MP3 carried any. ffmpeg passes -vn on every audio target here, which counts an embedded cover as a video stream and drops it along with the film it was never part of. That is deliberate rather than careless: keeping it makes the mux fail outright on several audio containers, and hiding the choice would be lying by omission.
Any hope of AAC sounding better than the MP3. Nothing in this pass adds information, and a bitrate higher than the MP3 deserved only makes a larger file of the same recording.
If a model is going to read the AAC
An AAC file is a waveform, and a language model reads text, so handing an assistant this AAC directly gets you nothing. The AAC becomes useful the moment somebody turns the sound into words, and that is a separate job with its own page here. MP3 to AAC is the step before it: get the sound out of the MP3 in a container the transcriber accepts, then hand the model a transcript instead of a file it cannot open.
Running MP3 to AAC against the real engine with a real file, ffmpeg wrote 27,216 bytes of AAC in 0.2 seconds, machine otherwise idle. That is one MP3 file on one day and not an average, which is why the number is given together with the file that produced it.
The MP3 to AAC 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 AAC file has anything inside it.
The same MP3 file, sent somewhere else
Other outputs the probe measured out of an MP3 file, so the cost of choosing AAC can be read against something. Sizes do not compare across engines, because each family was probed with its own MP3 witness file.
MP3 to WAV: 264,678 bytes in 0.1 seconds, verified by ffprobe.
MP3 to FLAC: 155,090 bytes in 0.1 seconds, verified by ffprobe.
MP3 to OGG: 8,752 bytes in 0.2 seconds, verified by ffprobe.
MP3 to M4A: 27,630 bytes in 0.2 seconds, verified by ffprobe.
MP3 to OPUS: 27,107 bytes in 0.2 seconds, verified by ffprobe.
MP3 to WMA: 54,944 bytes in 0.1 seconds, verified by ffprobe.
Other ways into AAC, and what they measured
Among the published routes into AAC, MP3 is the number 16 largest output of the 16 measured. The witness files differ, so this ranks the probe run and not your document.
3GP to AAC: 27,317 bytes in 0.2 seconds.
AVI to AAC: 27,485 bytes in 0.2 seconds.
FLAC to AAC: 27,219 bytes in 0.2 seconds.
FLV to AAC: 27,485 bytes in 0.2 seconds.
M4A to AAC: 27,317 bytes in 0.2 seconds.
What we will not pretend about MP3 to AAC
An MP3 file over 150 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
An MP3 to AAC 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 MP3 file behind the measurement on this page was a 3 second sine wave at 440 hertz, built by ffmpeg for the probe itself. That is why MP3 to AAC 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.
MP3 to AAC costs half a credit per conversion, whatever the length of the MP3. 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 MP3 longer than 60 minutes is refused before encoding. ffprobe measures the MP3 first and answers with a refusal, rather than starting a job the 240 second guard would kill halfway to the AAC.
MP3 to AAC 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 AAC that needed less than a second was answered with a refusal after twenty.
Nothing here reads what is inside the MP3. No transcription, no scene detection, no summary: MP3 to AAC is a container and codec operation, and the AAC says exactly what the MP3 said, in a different wrapper. The sound is the whole payload.
One MP3 file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
MP3 to AAC: what people ask
What actually converts my MP3 file to AAC?
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 MP3 file is uploaded once, ffmpeg runs once, the AAC comes back, and neither file is kept afterwards.
How long does MP3 to AAC take?
The probe measured 0.2 seconds for 27,216 bytes of AAC, 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 MP3 to AAC?
The one to know about first: Generation loss, and it is the whole story of this pair. MP3 threw information away to be small, AAC throws more away on top of what is left, and neither pass can restore what the other removed. Converting the AAC back to MP3 afterwards makes it worse a third time, not better.
Will the AAC sound better than the MP3?
No. Nothing in this pass adds information to the MP3, so the AAC is at best an exact carry of what you sent and at worst one more lossy pass over it. Asking for a higher bitrate than the MP3 deserved just makes a bigger file of the same recording.
Is MP3 to AAC free?
There is a free allowance every month, and one MP3 to AAC conversion costs half a credit against it however long the MP3 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.