M4A vs AAC: Are They the Same? The Difference Explained

M4A and AAC get used as if they meant the same thing, and most of the time the difference does not matter. But they describe different layers of a file, and mixing them up explains a lot of small mysteries: why two M4A files of the same song can differ several-fold in size, why a .aac file shows no cover art, and why some converters seem to do nothing. This guide explains what each one is, when the difference matters and how to convert between them.

The short answer

• AAC (Advanced Audio Coding) is a codec: the method that compresses the audio.
• M4A is a file type: an MPEG-4 file used for audio only.
• Most .m4a files hold AAC audio. Some hold Apple Lossless (ALAC) instead.
• A .aac file usually holds the same kind of AAC audio as a bare stream, without the MPEG-4 wrapper.
• Sound quality comes from the AAC encoding, not from the file type.

If the codec vs container idea is new, our guide to container vs codec explains it in full.

What AAC is

AAC is a lossy codec that MPEG standardized in 1997, first as part of MPEG-2 and later as part of MPEG-4. It was designed as the successor to MP3 and generally sounds better than MP3 at the same bitrate. Like MP3, it uses a model of human hearing to drop sound you are unlikely to notice.

AAC comes in a few profiles. AAC-LC (Low Complexity) is the standard one used for most music. HE-AAC and HE-AAC v2 add tricks that help at low bitrates, which is why they turn up in internet radio and streaming at modest bitrates.

AAC audio can be stored in several kinds of file:

• .m4a / .mp4 – the MPEG-4 container, the most common home for AAC
• .aac – a bare AAC stream in ADTS format, with very little around it
• .3gp – an older mobile container
• .mov and .mkv – video containers that often carry an AAC soundtrack

What M4A is

M4A is an MPEG-4 file used for audio only. MPEG-4 Part 14, the MP4 format, is based on Apple's QuickTime format, so .m4a, .mp4 and .mov files share the same basic structure of nested boxes. The .m4a extension signals "audio only," so players and file managers do not treat it as a video. The audio inside is stored the same way as the soundtrack of an .mp4 video, which is why saving the sound from an MP4 as an .m4a file can keep the audio unchanged.

Two related extensions are worth knowing:

• .m4b is used for audiobooks and podcasts, and supports chapters.
• .m4p marks protected files: tracks sold by the iTunes Store with FairPlay copy protection.

An M4A can hold either of two codecs:

• AAC (lossy): the usual case, including iTunes Store purchases (256 kbps AAC).
• Apple Lossless (ALAC): the same .m4a extension, but every sample kept. These files are several times bigger than AAC ones.

To see which one you have, open the file in VLC and choose Tools > Codec Information, or use MediaInfo.

M4A vs .aac files

When the audio inside is the same, the two files sound identical. The difference is what the wrapper adds:

For a music library or anything you want to tag, .m4a is the better file. Bare .aac is common in streaming and broadcast, where audio is sent as a continuous stream.

Telling AAC and Apple Lossless apart

Both use .m4a, so look at the bitrate or file size. AAC music files are usually 128 to 320 kbps; iTunes Store downloads are 256 kbps, which is about 1.9 MB per minute. Apple Lossless keeps every sample of CD-quality audio (1,411 kbps uncompressed) and usually compresses it to roughly half to two-thirds of that size, so a CD-quality ALAC file typically runs at several hundred kbps and takes about 5 to 7 MB per minute. If a 4-minute song in .m4a is around 8 MB, it is almost certainly AAC. If it is 20 MB or more, it is probably lossless.

Common situations

• An app accepts .m4a but rejects .aac: the app wants the MPEG-4 file. Put the audio into an M4A with AAC to M4A.
• A .aac file shows no cover art or wrong duration: bare ADTS streams have little room for tags and no index. An .m4a copy fixes both.
• Your M4A library is much bigger than expected: some files are probably Apple Lossless. Check the codec before assuming something went wrong.
• A car stereo skips your .m4a files: many older units read only MP3 (and sometimes WMA). Convert those files to MP3 at 256 or 320 kbps.
• Audio extracted from an MP4 video: the soundtrack is usually already AAC, so saving it as .m4a keeps the same kind of audio.

Sound quality

The file type has no effect on quality. A 256 kbps AAC stream sounds exactly the same in an .m4a, .mp4 or .aac file. What changes the sound is the AAC encoding: mainly the bitrate, the profile and the quality of the encoder.

As a rough guide, 128 kbps AAC is fine for casual listening, 256 kbps (the iTunes Store standard) is very hard to tell from the original for most listeners, and Apple Lossless removes the question entirely at the cost of space. For lossy vs lossless, see AAC vs FLAC.

Compatibility

• Apple devices and apps: both AAC in M4A and Apple Lossless in M4A play natively. M4A is Apple's standard file type for music.
• Android: Google's supported-formats list includes AAC in .mp4/.m4a and raw ADTS .aac files, so both play.
• Windows and macOS: modern versions play M4A natively.
• Web browsers: AAC in MP4 is, as MDN puts it, supported by all major browsers.
• Car stereos and older players: support varies. Many read MP3; some also read AAC or M4A. Check the manual, or convert to MP3 to be safe.

Which should you use?

In short, keep music as .m4a unless something specifically asks for a bare .aac file. The .m4a version holds the same audio and adds tags, artwork and accurate seeking, so it is the more useful file to keep.

Converting between M4A, AAC and other formats

• AAC to M4A: our AAC to M4A converter puts the audio into an M4A file you can tag and add to a library. It re-encodes the audio, so choose a high bitrate. If you need a bit-exact copy, FFmpeg can move the stream without re-encoding using -c copy.
• M4A to AAC: use M4A to AAC when a device or workflow wants a bare .aac file.
• For devices that do not play either: convert M4A to MP3 or AAC to MP3. Since both are lossy, use 256 or 320 kbps.
• For editing: decode to WAV with M4A to WAV or AAC to WAV.

These converters run in your browser with FFmpeg and never upload your files. Free users get 5 conversions per month. The output is limited to the first 10 seconds as a preview, with a 20MB input file size limit. Upgrade to Pro for unlimited, full-length conversions.

The bottom line

AAC is the audio; M4A is the box it usually comes in. For everyday listening they are interchangeable, because the same AAC stream sounds the same in any file. The difference matters in three cases: when an M4A turns out to hold lossless ALAC, when a bare .aac file lacks tags and cover art, and when an older device reads one file type but not the other. In those cases, check the codec first, then convert only as much as you need.