FLAC is lossless compression that hands back every sample it was given, at roughly half the size of the WAV it came from. 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 FLAC file on the way to becoming an AAC one.
What runs when a FLAC file becomes AAC
ffmpeg held to a single thread, on a machine we rent and watch. Your FLAC file is uploaded once, ffmpeg runs once, the AAC comes back, and neither file is kept. FLAC 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 FLAC into the AAC
The running time and the channel layout. A stereo FLAC 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 FLAC.
The whole of the FLAC, start to end. AAC is a lossy encoding, but it encodes all of the FLAC rather than a shortened or quieter version of it, and the loudness you hear is the loudness you sent.
What FLAC to AAC costs you
Exactness, once and for good. The FLAC holds every sample; AAC keeps what its model believes you can hear and discards the rest. Going back from AAC to FLAC later rebuilds the file size and none of the sound, which is the single most common misunderstanding about this pair.
Album art, where the FLAC 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 FLAC. Nothing in this pass adds information, and a bitrate higher than the FLAC 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. FLAC to AAC is the step before it: get the sound out of the FLAC in a container the transcriber accepts, then hand the model a transcript instead of a file it cannot open.
Running FLAC to AAC against the real engine with a real file, ffmpeg wrote 27,219 bytes of AAC in 0.2 seconds, machine otherwise idle. That is one FLAC file on one day and not an average, which is why the number is given together with the file that produced it.
The FLAC 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 FLAC file, sent somewhere else
Other outputs the probe measured out of a FLAC 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 FLAC witness file.
FLAC to MP3: 24,468 bytes in 0.1 seconds, verified by ffprobe.
FLAC to WAV: 264,678 bytes in 0.1 seconds, verified by ffprobe.
FLAC to OGG: 8,670 bytes in 0.1 seconds, verified by ffprobe.
FLAC to M4A: 27,633 bytes in 0.2 seconds, verified by ffprobe.
FLAC to OPUS: 27,591 bytes in 0.1 seconds, verified by ffprobe.
FLAC 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, FLAC is the number 14 largest output of the 16 measured. The witness files differ, so this ranks the probe run and not your document.
FLV to AAC: 27,485 bytes in 0.2 seconds.
M4A to AAC: 27,317 bytes in 0.2 seconds.
MKV to AAC: 27,699 bytes in 0.2 seconds.
MOV to AAC: 27,317 bytes in 0.2 seconds.
MPG to AAC: 27,223 bytes in 0.2 seconds.
What we will not pretend about FLAC to AAC
A FLAC file over 150 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
A FLAC 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 FLAC 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 FLAC 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.
FLAC to AAC costs half a credit per conversion, whatever the length of the FLAC. 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 FLAC longer than 60 minutes is refused before encoding. ffprobe measures the FLAC first and answers with a refusal, rather than starting a job the 240 second guard would kill halfway to the AAC.
FLAC 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 FLAC. No transcription, no scene detection, no summary: FLAC to AAC is a container and codec operation, and the AAC says exactly what the FLAC said, in a different wrapper. The sound is the whole payload.
One FLAC file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
FLAC to AAC: what people ask
What actually converts my FLAC 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 FLAC file is uploaded once, ffmpeg runs once, the AAC comes back, and neither file is kept afterwards.
How long does FLAC to AAC take?
The probe measured 0.2 seconds for 27,219 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 FLAC to AAC?
The one to know about first: Exactness, once and for good. The FLAC holds every sample; AAC keeps what its model believes you can hear and discards the rest. Going back from AAC to FLAC later rebuilds the file size and none of the sound, which is the single most common misunderstanding about this pair.
Will the AAC sound better than the FLAC?
No. Nothing in this pass adds information to the FLAC, 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 FLAC deserved just makes a bigger file of the same recording.
Is FLAC to AAC free?
There is a free allowance every month, and one FLAC to AAC conversion costs half a credit against it however long the FLAC 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.