MP3 or M4A: which should I save the sound from my video as?

The short comparison

• M4A is an audio-only MPEG-4 file. MDN lists MP4 as a file type that holds AAC sound. When the video's sound is already AAC, our tool copies it into the M4A file as it is.
• MP3 is, in MDN's words, "by far the most widely used and best-known" of the MPEG audio formats. Our tool makes MP3 files at 192 kbps. That is 192 thousand bits for every second of sound, or about 24 KB a second.
• AAC was, according to MDN, designed to give more compression with higher audio fidelity than MP3. MDN also says AAC support is "somewhat less predictable", and gives Firefox as an example.
• A copy is not a new conversion. FFmpeg's documentation describes its copy mode as streamcopy "with no decoding or encoding".

What we tested

We ran our Extract audio tool twice on the same 6-second test video that we generated ourselves. Its sound is stereo AAC at 162 kbps. We measured each result with ffprobe on 2026-10-13.

• M4A: 124 KB (123,927 bytes), made in about a tenth of a second. The sound was still AAC at 162 kbps, the same as in the video, and 6.00 seconds long.
• MP3: 145 KB (145,320 bytes), made in about 2 seconds. The sound was MP3 at 191 kbps, and 6.07 seconds long.

The M4A was about 15% smaller. That is because the M4A follows the video's own sound, which here was 162 kbps, while the MP3 follows our 192 kbps setting. With a video whose sound is stored at a higher rate, the result would differ.

On an iPhone running iOS 18.7, our own validation run on 2026-09-20 turned a 33-second recording into a 0.60 MB M4A in 0.03 seconds. We did not run MP3 on the iPhone.

What we did not test

We did not listen to the files or compare sound quality. We only measured file sizes, formats and lengths. We also did not test which players open each file, so "widely used" is MDN's description, not our measurement.

How to choose

1. If you want the sound exactly as the video stores it, and a small file, choose M4A. This applies when the video's sound is AAC.
2. If the file must work in as many places as possible, choose MP3.
3. Either way, you can make it with Extract audio from video. It runs in your browser, and your video is not uploaded.

More plain answers are in all guides.