WMA is Windows Media Audio, lossy, and tied to a decade when Windows Media Player ripped everybody CDs. 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 a WMA file on the way to becoming an AAC one.
What runs when a WMA file becomes AAC
ffmpeg held to a single thread, on a machine we rent and watch. Your WMA file is uploaded once, ffmpeg runs once, the AAC comes back, and neither file is kept. WMA 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 WMA into the AAC
The running time and the channel layout. A stereo WMA 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 WMA.
Everything the WMA still had. ffmpeg decodes the WMA in full before it writes a single byte of AAC, so nothing is skipped on the way in, whatever was already missing.
What WMA to AAC costs you
Generation loss, and it is the whole story of this pair. WMA 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 WMA afterwards makes it worse a third time, not better.
Album art, where the WMA 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 WMA. Nothing in this pass adds information, and a bitrate higher than the WMA 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. WMA to AAC is the step before it: get the sound out of the WMA in a container the transcriber accepts, then hand the model a transcript instead of a file it cannot open.
Running WMA to AAC against the real engine with a real file, ffmpeg wrote 27,510 bytes of AAC in 0.1 seconds, machine otherwise idle. That is one WMA file on one day and not an average, which is why the number is given together with the file that produced it.
The WMA 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 WMA file, sent somewhere else
Other outputs the probe measured out of a WMA 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 WMA witness file.
WMA to MP3: 24,259 bytes in 0.1 seconds, verified by ffprobe.
WMA to WAV: 262,222 bytes in 0.1 seconds, verified by ffprobe.
WMA to FLAC: 146,546 bytes in 0.1 seconds, verified by ffprobe.
WMA to OGG: 8,667 bytes in 0.1 seconds, verified by ffprobe.
WMA to M4A: 27,998 bytes in 0.1 seconds, verified by ffprobe.
WMA to OPUS: 25,770 bytes in 0.2 seconds, verified by ffprobe.
Other ways into AAC, and what they measured
Among the published routes into AAC, WMA is the third largest output of the 16 measured. The witness files differ, so this ranks the probe run and not your document.
WMV to AAC: 27,510 bytes in 0.1 seconds.
MP4 to AAC: 27,317 bytes in 0.2 seconds.
MP3 to AAC: 27,216 bytes in 0.2 seconds.
3GP to AAC: 27,317 bytes in 0.2 seconds.
AVI to AAC: 27,485 bytes in 0.2 seconds.
What we will not pretend about WMA to AAC
A WMA file over 150 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
A WMA 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 WMA 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 WMA to AAC came out at 0.1 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.
WMA to AAC costs half a credit per conversion, whatever the length of the WMA. 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.
A WMA longer than 60 minutes is refused before encoding. ffprobe measures the WMA first and answers with a refusal, rather than starting a job the 240 second guard would kill halfway to the AAC.
WMA 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 WMA. No transcription, no scene detection, no summary: WMA to AAC is a container and codec operation, and the AAC says exactly what the WMA said, in a different wrapper. The sound is the whole payload.
One WMA file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
WMA to AAC: what people ask
What actually converts my WMA 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 WMA file is uploaded once, ffmpeg runs once, the AAC comes back, and neither file is kept afterwards.
How long does WMA to AAC take?
The probe measured 0.1 seconds for 27,510 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 WMA to AAC?
The one to know about first: Generation loss, and it is the whole story of this pair. WMA 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 WMA afterwards makes it worse a third time, not better.
Will the AAC sound better than the WMA?
No. Nothing in this pass adds information to the WMA, 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 WMA deserved just makes a bigger file of the same recording.
Is WMA to AAC free?
There is a free allowance every month, and one WMA to AAC conversion costs half a credit against it however long the WMA 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.