What the converter does with your video
An MP4 file is a container: it holds a video track, usually an audio track, and information about how they fit together. MOV, M4A and WebM files work the same way. The converter opens the container in your browser, decodes the audio track and encodes that sound as an MP3. The pictures are left behind, so the result is an audio file only.
If the file has no audio track, for example a screen recording made without sound, the converter says so clearly instead of producing an empty MP3. The work is usually much faster than real time, so a long recording takes far less time to convert than to play.
Files are handled one at a time, up to 1 GB each. Because nothing is uploaded, even a large video never has to travel over your connection, and private recordings such as family videos, meetings or interviews stay on your device.
Choosing a bitrate
MP3 is a lossy format, and the bitrate decides how much data each second of sound may use. Higher bitrates sound closer to the source and produce bigger files. The three options cover the common needs:
| Bitrate | Approximate size per minute | Good for |
|---|---|---|
| 128 kbps | About 1 MB | Speech: lectures, podcasts, interviews, voice notes |
| 192 kbps (default) | About 1.4 MB | Everyday listening; a sensible balance for music and speech |
| 320 kbps | About 2.4 MB | Music, when you want the best quality MP3 can offer |
The sizes follow directly from the bitrate, so a one-hour lecture at 128 kbps comes to roughly 58 MB, and the same hour at 320 kbps to about 144 MB.
Why 320 kbps cannot beat the original soundtrack
The audio inside a video is almost always compressed already, commonly as AAC. Extracting it to MP3 means decoding that audio and compressing it again with a different lossy method, so the MP3 is a copy of a copy. A higher bitrate keeps that second step as faithful as possible, but no setting can add detail the original soundtrack did not have.
That is why 320 kbps is the right choice for music you care about, and why converting a low-quality video at 320 kbps mainly gives you a larger file. For the same reason, avoid chains of conversions: if you have the original audio recording, use that rather than a video made from it.
MP4 and MP3 compared
| MP4 | MP3 | |
|---|---|---|
| Contents | Video and audio in one container | Audio only |
| Audio quality | Depends on how the video was encoded | Set by the bitrate you choose (128 to 320 kbps) |
| Typical size | Large, mostly because of the video | Roughly 1 to 2.4 MB per minute |
| Plays on | Video players, browsers, phones | Practically every audio player, phone and many car stereos |
| Good for | Watching | Listening, podcasts, audio editing |
When MP3 is not the answer
- You want a short moment as an animation: MP4 to GIF turns a few seconds of video into a looping GIF, without sound.
- You need the audio back as a video: some platforms accept only video files. MP3 to MP4 pairs an audio file with a still picture.
- You need untouched audio for professional editing: an MP3 is always a lossy re-encode, so work from the original recording if you have it.
- The recording is not yours: only convert material you have the right to use.