Audio Formats and Codecs Explained
Lossy vs lossless, codec vs container, and how MP3, AAC, Opus, FLAC, WAV, AC3, EAC3 and DTS compare, in plain English.
An audio format is just the way sound is stored in a file, and there are dozens of them. Most of the confusion comes from two questions: is the format lossy or lossless, and is it a codec or a container? Once those click, choosing between MP3, AAC, FLAC, WAV and the rest gets simple. This guide covers both, then compares the formats you're most likely to run into.
Codec vs Container: What an Audio Format Actually Is
A codec (short for coder/decoder) is the method used to squeeze audio into data and turn it back into sound. A container is the file wrapper that holds that data, along with things like the title, artist and cover art. Mozilla's audio codec guide puts it simply: codecs do the encoding and decoding, and codec data is stored inside a container.
Sometimes the two share a name, sometimes they don't:
- MP3 is both the codec and, in practice, the file type.
- M4A is a container that usually holds AAC, but can also hold Apple Lossless (ALAC). See AAC vs M4A.
- MKV and MP4 are video containers that can carry AAC, AC3, EAC3, DTS or Opus audio tracks.
- WAV is a container that almost always holds plain, uncompressed PCM audio.
That's why two ".m4a" files can behave differently, and why a video can play fine on one device and silently on another: the container is familiar, but the codec inside isn't.
Lossy vs Lossless Audio Compression
Audio compression, in the file-size sense, comes in two kinds.
Lossy codecs like MP3, AAC and Opus throw away sound the encoder expects you won't notice, based on how human hearing works. The files are small, often 80 to 95% smaller than the original according to Mozilla, but the discarded detail is gone for good. At higher bitrates (256 to 320 kbps for MP3), most people can't tell the difference in a blind test.
Lossless codecs like FLAC and ALAC shrink files without discarding anything. Decode a FLAC file and you get back exactly the original, bit for bit. The trade-off is size: lossless files typically end up around half the size of the uncompressed original, not a tenth. FLAC's own site describes it as the most widely supported lossless codec, and it's open and patent-free.
Uncompressed formats like WAV and AIFF don't compress at all. CD-quality stereo runs at about 1,411 kbps, roughly 10 MB a minute.
One note: "audio compression" can also mean dynamic range compression, the studio effect that narrows the gap between loud and quiet parts. That's a different thing entirely and has nothing to do with file size.
The Main Audio Formats Compared
| Format | Type | Typical bitrate or size | Best for |
|---|---|---|---|
| MP3 | Lossy | 128–320 kbps | Everything: phones, cars, old MP3 players |
| AAC (.m4a, .aac) | Lossy | 128–256 kbps | Apple devices, YouTube, streaming |
| Opus (.opus, .webm) | Lossy | 6–510 kbps | Voice calls, web audio, YouTube |
| Ogg Vorbis (.ogg) | Lossy | 45–500 kbps | Spotify, games, open-source software |
| WMA (.wma) | Lossy | 64–192 kbps | Older Windows libraries |
| FLAC (.flac) | Lossless | Roughly half of WAV | Archiving, hi-fi listening |
| ALAC (.m4a) | Lossless | Roughly half of WAV | Lossless in the Apple ecosystem |
| WAV (.wav) | Uncompressed | ~1,411 kbps at CD quality | Editing, recording, pro audio |
| AIFF (.aiff) | Uncompressed | ~1,411 kbps at CD quality | Editing on Mac |
| AC3 (Dolby Digital) | Lossy, surround | 448–640 kbps (5.1) | DVDs, broadcast TV |
| EAC3 (Dolby Digital Plus) | Lossy, surround | Lower than AC3 for the same quality | Streaming, 4K, Dolby Atmos |
| DTS | Lossy, surround (lossless in DTS-HD MA) | 768 kbps or 1.5 Mbps (5.1) | DVDs, Blu-rays, home theater |
| AMR (.amr) | Lossy, speech | Under 13 kbps | Phone voice recordings |
Bitrate ranges come from Mozilla's codec reference for the web formats and from the Dolby and DTS specs for the surround formats. Real files vary with the encoder and settings.
Formats for Music: MP3, AAC, Opus and Vorbis
MP3 is the universal fallback. It was finalized in 1992, its patents have expired, and every phone, car stereo and music player made in the last 25 years plays it. It tops out at 320 kbps. See the MP3 bitrate guide.
AAC is MP3's more efficient successor and Apple's default. A 256 kbps AAC file generally sounds as good as a 320 kbps MP3, which is why Apple Music and YouTube use it. Our AAC vs MP3 page goes deeper.
Opus is the newest of the group, standardized in 2012, and the most efficient at low bitrates. It's the codec behind a lot of voice calling and web audio, and it sounds very good for music at 96 to 160 kbps. Ogg Vorbis is its older open-source cousin, still used by games and some apps.
Lossless Formats: FLAC, ALAC, WAV and AIFF
If you're archiving music, editing audio or listening on a high-end setup, lossless is the way to go. FLAC is the standard for lossless downloads and plays almost everywhere except some Apple apps. ALAC does the same job inside the Apple ecosystem. WAV and AIFF skip compression entirely, which makes them easy for editing software to handle but bulky to store. See is FLAC lossless? and WAV vs FLAC.
Remember that lossless only preserves what went in. Converting an MP3 to FLAC gives you a bigger file, not better sound.
Surround Sound Formats: AC3, EAC3 and DTS
Movies and TV use their own family of formats built for 5.1 and 7.1 speakers. AC3 (Dolby Digital) is the classic, found on DVDs and broadcast TV. EAC3 (Dolby Digital Plus) is its more efficient successor, used by streaming services and as the carrier for Dolby Atmos on streaming. DTS is the main rival on DVD and Blu-ray. On disc you'll also meet the lossless versions: Dolby TrueHD and DTS-HD Master Audio.
These formats are the usual reason a video plays with no sound: the player can read the container but has no decoder for the Dolby or DTS track. Our AC3 vs AAC vs EAC3 guide covers the fixes, and the AC3 to MP3 converter turns those tracks into stereo files that play anywhere.
What an Audio Decoder Does
A decoder is the half of a codec that turns stored data back into sound. Your phone, browser, TV or media player has to include a decoder for every format it plays. When it doesn't, you get "format not supported," a file that won't open, or a video with no audio.
Decoders can be software (built into an app like VLC) or hardware (a chip in a TV, soundbar or AV receiver). Common formats like MP3 and AAC are supported almost everywhere. Licensed or niche formats, like EAC3, DTS or AMR, are where support gets patchy. The fix is either a player that includes the decoder or converting the file to a more common format.
Which Audio Format Should You Use?
- Maximum compatibility: MP3 at 320 kbps.
- iPhone, iPad and Apple Music: AAC at 256 kbps, or ALAC for lossless.
- Archiving a music collection: FLAC.
- Editing, recording or podcast production: WAV (or AIFF on Mac).
- Voice notes and calls: Opus or AAC at low bitrates.
- Home theater: keep the original AC3, EAC3 or DTS track.
Converting Between Formats
Converting is useful when a device won't play something, but it follows one rule: you can't add quality that isn't in the source. Lossy to lossless (MP3 to FLAC) makes a bigger file with the same sound. Lossless to lossy (FLAC to MP3) is fine and often sensible for portable devices. Lossy to lossy (AAC to MP3) loses a little more each time, so use a high bitrate.
Our free converters handle the common cases in your browser: the MP3 converter, AAC converter, FLAC converter, WAV converter and audio to MP3 for anything else.
Need a file in a different format? Upload it and convert to MP3, AAC, FLAC or WAV for free.
Open the free audio converter →Frequently Asked Questions
What is the difference between an audio format and a codec?
A codec is the method used to compress and decode audio, like AAC or FLAC. A format or container is the file type that holds it, like .m4a or .mkv. Some names cover both, such as MP3.
What is the best audio format?
It depends on the job. MP3 is best for compatibility, AAC for Apple devices, FLAC for archiving music without loss, and WAV for editing. For surround sound, keep the original AC3, EAC3 or DTS track.
Is FLAC better than MP3?
FLAC is lossless, so it keeps every detail of the source, while MP3 discards some to save space. At 320 kbps most people can't hear the difference, but FLAC is better for archiving and editing.
What is lossless audio compression?
Lossless compression makes a file smaller without throwing away any sound. When a FLAC or ALAC file is decoded, the audio is identical to the original, bit for bit.
What is an audio decoder?
The part of a codec that turns stored data back into sound. If a device lacks the decoder for a format, like EAC3 or DTS, the file won't play or plays without audio.
Which audio formats play on almost everything?
MP3 is the safest choice, followed by AAC. WAV also plays widely but makes large files. FLAC, Opus and surround formats like EAC3 have more limited support on older devices.