FLAC vs MP3 vs AAC vs Opus: lossless and lossy audio, explained

What lossless actually means, when it matters, which lossy format to pick for a phone or a car, and the conversions that make sense. With the one rule that saves people the most disk space.

formatsaudiocomparison

Audio formats generate more folklore than any other kind of file. People convert MP3s to FLAC to “upgrade” them, re-encode lossless files to lossy ones and lose the originals, and argue about bitrates nobody can hear. Most of it comes down to one distinction and a few facts about where each format plays.

Lossless means lossless

A lossless format stores the audio exactly. Decode it and you get back the identical samples that went in, bit for bit; the file is smaller than raw audio because the encoder finds mathematical patterns in the waveform, the way a ZIP finds patterns in text. FLAC, ALAC (Apple Lossless) and WAV are lossless; WAV is simply uncompressed, and the other two shrink it by 30 to 60 percent.

A lossy format discards sound the encoder’s model of human hearing judges inaudible, and then compresses what remains. The result is five to ten times smaller than lossless, and the discarded sound is gone forever. MP3, AAC, Ogg Vorbis, Opus and WMA are lossy.

That is the whole distinction. Everything else is about which lossy format and which bitrate.

The rule that saves the most space and the most regret

Keep one lossless master. Make lossy copies from it, never from another lossy file.

Lossy to lossy conversion is where quality actually dies: an MP3 converted to AAC carries both encoders’ losses, and each generation gets worse. Lossless to lossy, done once, at a sensible bitrate, is inaudible to nearly everyone in nearly every listening situation.

The reverse, lossy to lossless, is pointless. An MP3 converted to FLAC is a large file containing exactly the MP3’s sound and nothing more. The detail was discarded when the MP3 was made and no format can put it back.

FLAC: the archival format

Open, patent-free, standardised as an RFC in 2024, and supported by nearly every player, phone and streamer. It carries full tags, embedded artwork and an integrity hash. This is the format to rip CDs into and the one lossless services (Qobuz, Tidal, Amazon) deliver.

Its one practical gap is Apple’s Music app, which does not import FLAC. Convert to ALAC, which is lossless both ways, if the library has to live there.

MP3: the one that plays anywhere

MP3 is a 1993 format with 1993 compression. At 320 kbps it is fine; at 128 kbps it is audibly worse than every newer codec at the same bitrate. Its patents expired in 2017 and nothing refuses to play it, which is its entire remaining advantage. For a car stereo from 2008, a cheap Bluetooth speaker or a stranger’s unknown device, MP3 is still the answer.

AAC: the Apple and streaming default

AAC is what MP3 should have been: the same job, around 30 percent more efficient. It is the format of iTunes, Apple Music, YouTube and most streaming. In an .m4a container it plays on every Apple device, every Android phone, every browser and nearly every car. Where MP3 is the lowest common denominator, AAC is the sensible default.

Opus: the best codec, in fewer places

Opus (2012) is the current state of the art: at 96 kbps it matches what AAC needs 128 and MP3 needs 192 to achieve, and it is unmatched for speech. It is royalty-free and standardised by the IETF. Every browser, Discord, WhatsApp and Signal use it, and Android and recent Apple systems play it, but hardware players, cars and older devices mostly do not, and tagging support in music players is patchy. Excellent for streaming and voice; still a slightly awkward choice for a music library.

The ones to migrate away from

Ogg Vorbis was the open alternative to MP3 for a decade and has been superseded by Opus, from the same organisation. Files play fine; make new ones in Opus.

WMA is Microsoft’s 1999 format, with no native playback on Apple devices and a share of DRM-locked files that can never be unlocked. Convert any WMA Lossless to FLAC (lossless), and leave standard WMA as it is until you can re-rip the source.

Bitrates, briefly

For lossy music that will be listened to rather than studied:

  • AAC or Opus at 160 to 192 kbps: transparent to almost everyone on almost everything.
  • MP3 at 256 to 320 kbps (or -V 0 variable bitrate): the same result, in a less efficient format.
  • Anything above these is wasting space on a lossy file; if the space is acceptable, keep lossless instead.

Conversions

# CD rip or WAV -> FLAC (best compression; decoding speed is unaffected)
flac -8 track.wav

# FLAC -> AAC for Apple devices
ffmpeg -i track.flac -c:a aac -b:a 192k track.m4a

# FLAC -> Opus for everything modern
ffmpeg -i track.flac -c:a libopus -b:a 160k track.opus

# FLAC -> MP3 for everything old
ffmpeg -i track.flac -c:a libmp3lame -q:a 0 track.mp3

# FLAC -> ALAC, lossless, for Apple Music
ffmpeg -i track.flac -c:a alac track.m4a

# Verify a FLAC against its embedded checksum
flac -t track.flac

Which format you actually have

Extensions lie here as much as anywhere: .wav files that are really MP3 inside a WAV wrapper, .m4a files that are ALAC rather than AAC, .flac files that were transcoded from MP3. The container signature (fLaC, RIFF....WAVE, ID3 or FF FB for MP3, ftypM4A) says what the file is; ffprobe or the detector says it without guessing. If a lossless file sounds like a lossy one, a spectrogram with nothing above 16 kHz is the usual tell that it came from an MP3.

The decision, in three lines

  • Keeping music: FLAC (or ALAC if it lives in Apple Music).
  • Carrying it on a phone or streaming it: AAC or Opus at 160 to 192 kbps.
  • Sending it to an unknown device: MP3 at 320 kbps, and accept the size.
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