Opus is the lossy encoding voice calls and browsers settled on, better than MP3 at the same bitrate and unreadable by most hardware players. FLAC is lossless compression that hands back every sample it was given, at roughly half the size of the WAV it came from. ffmpeg sits between the two, and this page says exactly what happens to an OPUS file on the way to becoming a FLAC one.
What runs when an OPUS file becomes FLAC
ffmpeg held to a single thread, on a machine we rent and watch. Your OPUS file is uploaded once, ffmpeg runs once, the FLAC comes back, and neither file is kept. OPUS to FLAC 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 FLAC
The running time and the channel layout. A stereo OPUS comes back as a stereo FLAC 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 FLAC, so nothing is skipped on the way in, whatever was already missing.
What OPUS to FLAC costs you
Nothing further, and nothing recovered either. FLAC stores every sample it is handed faithfully, but the samples it is handed came out of an OPUS decoder. The FLAC is an exact copy of an approximation, and it is several times the size of the OPUS for the same sound.
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 FLAC 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 FLAC
A FLAC file is a waveform, and a language model reads text, so handing an assistant this FLAC directly gets you nothing. The FLAC becomes useful the moment somebody turns the sound into words, and that is a separate job with its own page here. OPUS to FLAC 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 FLAC against the real engine with a real file, ffmpeg wrote 178,357 bytes of FLAC in 0.1 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 FLAC 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 FLAC 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 FLAC 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 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 AAC: 27,353 bytes in 0.2 seconds, verified by ffprobe.
OPUS to WMA: 58,144 bytes in 0.1 seconds, verified by ffprobe.
Other ways into FLAC, and what they measured
Among the published routes into FLAC, OPUS is the first largest output of the 16 measured. The witness files differ, so this ranks the probe run and not your document.
WEBM to FLAC: 178,357 bytes in 0.2 seconds.
WMA to FLAC: 146,546 bytes in 0.1 seconds.
WMV to FLAC: 146,546 bytes in 0.1 seconds.
WAV to FLAC: 44,337 bytes in 0.1 seconds.
MP4 to FLAC: 156,949 bytes in 0.1 seconds.
What we will not pretend about OPUS to FLAC
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 FLAC 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 FLAC 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.
OPUS to FLAC 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 FLAC.
OPUS to FLAC 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: a FLAC 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 FLAC is a container and codec operation, and the FLAC 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 FLAC: what people ask
What actually converts my OPUS file to FLAC?
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 FLAC comes back, and neither file is kept afterwards.
How long does OPUS to FLAC take?
The probe measured 0.1 seconds for 178,357 bytes of FLAC, 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 FLAC?
The one to know about first: Nothing further, and nothing recovered either. FLAC stores every sample it is handed faithfully, but the samples it is handed came out of an OPUS decoder. The FLAC is an exact copy of an approximation, and it is several times the size of the OPUS for the same sound.
Will the FLAC sound better than the OPUS?
No. Nothing in this pass adds information to the OPUS, so the FLAC is at best an exact carry of what you sent. Asking for a higher bitrate than the OPUS deserved just makes a bigger file of the same recording.
Is OPUS to FLAC free?
There is a free allowance every month, and one OPUS to FLAC 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.