Free converter · Extract audio
Convert MP4 to MP3
Short answer
Drop your MP4 file into the converter below and download the MP3 file. It runs in your browser with FFmpeg, for files up to 100 MB; nothing is uploaded. Only the sound is kept: the video track is dropped and the audio is saved as MP3 audio at about 128 kbps, keeping the source's sample rate.
Maximum file size: 100 MB. The first conversion loads a ~30 MB converter engine, once per visit. Nothing is uploaded - conversion runs on your device.
Extract audio from MP4 into MP3. · Extract audio · Browser-side via FFmpeg WASM - nothing uploaded.
Key takeaways
- The video is dropped and the audio is re-encoded to MP3 with ffmpeg's default libmp3lame settings: 128 kbps, constant bitrate.
- Measured: 10 seconds of stereo became 160,852 bytes, which is 128 kbps (128 kbps × 10 s ÷ 8 = 160,000 bytes plus framing).
- If the MP4's audio was AAC at a higher bitrate, the MP3 is a step down in both size and quality.
- For better quality use -q:a 2 or -b:a 192k; the FFmpeg wiki says -q:a 0 to 3 normally produce transparent results.
Measured: what you get
Measured on the soundtrack of a 10 second excerpt of Big Buck Bunny (animated film; (c) Blender Foundation, CC BY 3.0), stereo, 48 kHz, using the same ffmpeg arguments this converter passes to its WebAssembly build, run with ffmpeg 6.1.1. Results describe this file.
| Check | Result |
|---|---|
| Input | MP4, 9,181,123 bytes, AAC 192 kbps stereo |
| Output | MP3, 160,852 bytes, 128 kbps CBR, 48 kHz stereo |
| Time, native ffmpeg | 0.29 s |
| Time, converter's WebAssembly core, 3 s clip (mov source) | 0.1 s |
Computed: size in bytes = bitrate in bits per second × seconds ÷ 8. A 3.5 minute (210 s) song is 3.36 MB at 128 kbps, 5.04 MB at 192 kbps, 6.72 MB at 256 kbps and 8.4 MB at 320 kbps.
MP4 vs MP3
| MP4 | MP3 | |
|---|---|---|
| Full name | MPEG-4 Part 14 | MPEG-1 Audio Layer III |
| What is inside | H.264 or H.265 video with AAC audio | MP3 audio |
| Compression | Depends on the codec inside | Lossy |
| Best for | Uploading anywhere: the format social platforms document first | Music and podcasts that must play on every device |
What you get
MP3 audio at about 128 kbps, keeping the source's sample rate. Extract audio from MP4 into MP3.
Choosing quality yourself
ffmpeg -i in.mp4 -vn -codec:a libmp3lame -qscale:a 2 out.mp3is variable bitrate; the FFmpeg wiki says the range is 0 to 9, lower is better, 0 to 3 normally transparent, 4 the default.ffmpeg -i in.mp4 -vn -b:a 192k out.mp3sets a constant bitrate. The wiki calls 320k generally wasteful.- Multi-channel audio is mixed down to stereo, because MP3 here carries at most two channels.
When to avoid MP3
MP3 is a second lossy generation when the video's audio is AAC. If you only need the sound for editing or archiving, extract the AAC as it is (-c:a copy to .m4a) or write WAV.
Questions people ask
Is this MP4 to MP3 converter free and private?
Yes. It is free, needs no account and runs entirely in your browser, so the file never leaves your device.
Will converting MP4 to MP3 reduce quality?
The audio is re-encoded to MP3, a lossy format, so there is a small loss compared with the original track.
Is there a file size limit?
Files up to 100 MB. The first conversion downloads a converter engine of about 30 MB once per visit; larger files are better handled by a desktop app such as HandBrake or FFmpeg.
What bitrate does this MP4 to MP3 converter use?
ffmpeg's libmp3lame default: 128 kbps constant bitrate. Our test file measured exactly 128 kbps.
Will MP3 sound worse than the MP4's audio?
If the MP4 had AAC at 192 kbps, as in our test, a 128 kbps MP3 is lower in bitrate and a second lossy step.
How big is an MP3 of a 3.5 minute video?
About 3.36 MB at 128 kbps (computed).
Can I keep surround sound?
No. MP3 output is mixed down to stereo; our 6-channel source produced a 2-channel MP3.
More converters
Sources and how we checked
- MDN: Media container formats
- MDN: Web audio codec guide
- MDN: Image file type and format guide
- FFmpeg documentation: stream copy
- FFmpeg Documentation: ffmpeg (stream selection, -map, -c copy) (read 2026-10-08)
- FFmpeg Wiki: Encode/MP3 (libmp3lame quality and bitrate) (read 2026-10-08)
- Blender Foundation: Big Buck Bunny (CC BY 3.0), the clip used for the measurements (read 2026-10-08)
Reviewed October 8, 2026. What the converter writes was measured with ffprobe on the same FFmpeg build your browser downloads; format descriptions follow MDN's media guides.