Convert MP3 to AAC

.aac or .m4a: the difference

The sound is identical: same codec, same bit rate, same size within a few kilobytes. Pick .m4a unless something specifically asks for .aac, such as a car stereo or older player whose manual lists .aac but not .m4a, or a tool that wants a raw AAC stream to combine with video. The name ADTS, which you may see in technical specs, simply means this frame-by-frame layout.

Changing your mind later

Because both hold the same AAC sound, moving between them needs no second encode. Convert an .aac file to M4A, or an .m4a to AAC, with As it is, and the sound is copied straight across; the row reports that nothing was re-encoded. It is instant and loses nothing. So if you made .aac files and an app later wants .m4a, there's no reason to go back to the MP3s. It is also the quickest fix when a phone or car stereo shows the wrong length for a bare .aac file: the .m4a has an index, so its length is known exactly.

The one thing a bare .aac can't pass on is tags: an .m4a made from it starts without a title or cover, so if names and artwork matter, convert the MP3s to M4A directly. The reverse trip, M4A to .aac, drops the tags along the way, since a bare stream has nowhere to keep them.

Quality from an MP3

Going from MP3 to AAC means compressing already-compressed sound, so a little more detail goes. As it is keeps the loss low by copying the MP3's bit rate, within the AAC presets of 128 to 256 kbps: a 192 kbps MP3 becomes 192 kbps AAC, a 320 kbps one 256. AAC is the more efficient codec, so 256 kbps holds up well against a 320 kbps original.

Avoid converting to a much lower rate in the hope that AAC will make up for it. It can't recover what the lower rate leaves out; use Small only when size matters more than sound, as with speech. Mono MP3s get half the rate, as low as 64 kbps, and sample rates up to 48 kHz are kept as they are. To hear the difference for yourself, convert one song at High and one at Balanced and play both on the device you'll use.

Where AAC goes next

AAC is the usual sound inside MP4 videos, so an .aac or .m4a fits naturally under a video, and Octoolo's Video Editor takes either as a music track; the add music to a video page walks through it. AAC is also what iPhones and Android phones play natively and what iTunes uses for songs.

Going the other way, from AAC back to MP3 for an older device, is covered on AAC to MP3, and MP3 to M4A has more on tags and Apple devices. Your MP3s stay where they were in every case, so you can always start again from them.

Need the AAC sound of a video rather than of an MP3? Convert sound files takes videos too, and with As it is an MP4's AAC track is copied out without being re-encoded; see MP4 to M4A.