In this guide
AAC is a family of lossy audio codecs, while M4A usually names an audio file using MPEG-4 packaging. An AAC recording can be stored in an .aac file or an M4A file. Check the file and destination before converting, and keep the original.
AAC stands for Advanced Audio Coding, a family of lossy audio codecs used to compress sound. A codec describes how audio is encoded and decoded. AAC can make a compact listening file by discarding some audio information; the resulting quality depends on the source, encoder and settings.
AAC audio can appear in more than one kind of file. For example, MDN's audio codec guide lists AAC with MP4 and ADTS packaging. An .aac extension often identifies an AAC stream, while an .m4a extension commonly identifies audio in an MPEG-4 container. The player needs to support the packaging and the particular AAC profile.
AAC vs. M4A: codec or file?
| Term | What it means | Practical implication |
|---|---|---|
| AAC | A family of lossy audio codecs. | It describes the encoding, not a single filename or container. |
| .aac | An extension used for AAC audio, often in an ADTS stream. | Check that the player can read this packaging and decode the file's profile. |
| .m4a | Commonly an audio file using MPEG-4 packaging. | It may contain AAC or another codec, such as Apple Lossless; the extension alone does not prove which. |
Think of a container as packaging and a codec as the method used for the sound inside. See what an M4A file is for more on MPEG-4 audio. Renaming a file changes its name; it does not convert the audio or reliably change its packaging.
Is AAC good quality?
AAC can provide useful listening quality at a compact size, but its name or bitrate alone cannot guarantee how a particular recording sounds. Encoder settings, source quality, content and listening conditions all matter. AAC is lossy: once information has been discarded during encoding, a later conversion cannot recover it. Keep a higher-quality original when one is available.
Bitrate is a useful size guide, not a direct quality score. For example, one minute at a constant 192 kilobits per second contains approximately 1,440,000 bytes of audio data before file overhead. Variable bitrate, metadata and duration change the result. Use the audio file size calculator for another duration or bitrate.
AAC vs. MP3 vs. WAV
| Format | Useful for | Tradeoff |
|---|---|---|
| AAC | Compressed listening audio when the destination supports the file. | Lossy, with support varying by player, profile and packaging. |
| MP3 | A widely supported listening or sharing copy. | Also lossy; converting AAC to MP3 adds another lossy encode. |
| PCM WAV | An uncompressed working or delivery copy when required. | Often much larger, and cannot restore information lost in the AAC source. |
A WAV file is a container and is not always uncompressed; see the WAV guide. For a closer comparison with a common AAC container, read M4A vs. MP3.
How to open an AAC file
- Try a player or editor that supports the file's AAC profile and packaging. Listen to the start and end to check the whole recording.
- If it fails, confirm the download completed and inspect the codec and container with a compatible media inspector, or ask the sender for those details. Different players and browsers support different combinations.
- Try another compatible player or ask for an export accepted by your destination. Keep the source rather than changing its extension.
How to convert an AAC file to MP3 or WAV
- Open AAC to MP3 and select a local .aac file up to five minutes and 50 MiB. Your browser must decode it, and the decoded audio must be mono or stereo.
- Choose MP3 at 128, 192 or 320 kbps, or 16-bit PCM WAV at 44.1 or 48 kHz. The prepared output must also fit within 50 MiB.
- Prepare and preview the output, then download and test it in the destination app. Keep the original.
The browser decodes at 48 kHz, and conversion may resample audio. MP3 introduces another lossy encode; PCM WAV avoids that additional MP3 encode but cannot recover discarded information. Encoding delay or padding can affect timing, and the tool does not copy tags or artwork. If the browser cannot decode the file or it exceeds the limits, use a compatible desktop editor.
Have an .m4a file instead? Use M4A to MP3 for a supported local file, or read the M4A conversion guide first.
Common questions
Is AAC lossless?
No. AAC is lossy. An M4A file can instead contain a lossless codec such as Apple Lossless, so inspect its codec rather than assuming from the extension.
Is AAC the same as MP3?
No. Both are lossy audio codecs, but they use different encoding methods. Compatibility and results depend on the file, settings and destination.
Will converting AAC to WAV improve quality?
It cannot restore information discarded by AAC encoding. PCM WAV may be useful for an editing workflow, but resampling or changing bit depth can also alter the samples.
Can I rename an AAC file to MP3?
No. Changing the extension does not re-encode the sound. Use a compatible converter if the destination requires MP3, and check the resulting file.