Opus is the lossy encoding voice calls and browsers settled on, better than MP3 at the same bitrate and unreadable by most hardware players. 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 OPUS file on the way to becoming an AAC one.
What runs when an OPUS file becomes AAC
ffmpeg held to a single thread, on a machine we rent and watch. Your OPUS file is uploaded once, ffmpeg runs once, the AAC comes back, and neither file is kept. OPUS 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 OPUS into the AAC
The running time and the channel layout. A stereo OPUS 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 OPUS.
Everything the OPUS still had. ffmpeg decodes the OPUS in full before it writes a single byte of AAC, so nothing is skipped on the way in, whatever was already missing.
What OPUS to AAC costs you
Generation loss, and it is the whole story of this pair. OPUS 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 OPUS afterwards makes it worse a third time, not better.
Album art, where the OPUS 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 OPUS. Nothing in this pass adds information, and a bitrate higher than the OPUS 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. OPUS to AAC is the step before it: get the sound out of the OPUS in a container the transcriber accepts, then hand the model a transcript instead of a file it cannot open.
Running OPUS to AAC against the real engine with a real file, ffmpeg wrote 27,353 bytes of AAC in 0.2 seconds, machine otherwise idle. That is one OPUS file on one day and not an average, which is why the number is given together with the file that produced it.
The OPUS 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 OPUS file, sent somewhere else
Other outputs the probe measured out of an OPUS 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 OPUS witness file.
OPUS to MP3: 24,429 bytes in 0.2 seconds, verified by ffprobe.
OPUS to WAV: 288,078 bytes in 0.1 seconds, verified by ffprobe.
OPUS to FLAC: 178,357 bytes in 0.1 seconds, verified by ffprobe.
OPUS to OGG: 9,472 bytes in 0.2 seconds, verified by ffprobe.
OPUS to M4A: 27,734 bytes in 0.2 seconds, verified by ffprobe.
OPUS to WMA: 58,144 bytes in 0.1 seconds, verified by ffprobe.
Other ways into AAC, and what they measured
Among the published routes into AAC, OPUS is the seventh largest output of the 16 measured. The witness files differ, so this ranks the probe run and not your document.
WAV to AAC: 27,219 bytes in 0.2 seconds.
WEBM to AAC: 27,353 bytes in 0.2 seconds.
WMA to AAC: 27,510 bytes in 0.1 seconds.
WMV to AAC: 27,510 bytes in 0.1 seconds.
MP4 to AAC: 27,317 bytes in 0.2 seconds.
What we will not pretend about OPUS to AAC
An OPUS file over 150 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
An OPUS 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 OPUS 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 OPUS 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.
OPUS to AAC costs half a credit per conversion, whatever the length of the OPUS. 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 OPUS longer than 60 minutes is refused before encoding. ffprobe measures the OPUS first and answers with a refusal, rather than starting a job the 240 second guard would kill halfway to the AAC.
OPUS 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 OPUS. No transcription, no scene detection, no summary: OPUS to AAC is a container and codec operation, and the AAC says exactly what the OPUS said, in a different wrapper. The sound is the whole payload.
One OPUS file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
OPUS to AAC: what people ask
What actually converts my OPUS 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 OPUS file is uploaded once, ffmpeg runs once, the AAC comes back, and neither file is kept afterwards.
How long does OPUS to AAC take?
The probe measured 0.2 seconds for 27,353 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 OPUS to AAC?
The one to know about first: Generation loss, and it is the whole story of this pair. OPUS 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 OPUS afterwards makes it worse a third time, not better.
Will the AAC sound better than the OPUS?
No. Nothing in this pass adds information to the OPUS, 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 OPUS deserved just makes a bigger file of the same recording.
Is OPUS to AAC free?
There is a free allowance every month, and one OPUS to AAC conversion costs half a credit against it however long the OPUS 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.