Where WebM files come from
WebM is the format browsers record in. A screen recording made with a browser extension or a
web app, a meeting saved from a browser tab, a voice message recorded on a website and many
downloads from video sites arrive as .webm. Inside there is usually Opus sound
(sometimes the older Vorbis), with or without VP8 or VP9 video. Plenty of software still won't
open it: car stereos, older phones, some editors and most upload forms that ask for audio want
MP3.
Recordings with no length
Recordings made in a browser have a quirk that none of the converters we looked at mention. The browser writes the file while you record and never goes back to the beginning to note how long it turned out, so the file has no duration and no seek index. We recorded two files in Chrome with its built-in recorder, an 8-second audio clip and a screen-style recording with VP9 video, and opened each in Chrome's own player:
| Recording | Length Chrome's player reports | Real length | Length of the MP3 |
|---|---|---|---|
| Audio only, Opus 128 kbps | Infinity, no seeking | 7.02 s | 7.03 s |
| VP9 video + Opus sound | Infinity, no seeking | 7.98 s | 7.98 s |
The sound is all there; only the bookkeeping is missing. This tool reads the length from the audio packets themselves, and the MP3 it writes has a proper header, so players show its duration and you can jump to any point. When a WebM is missing its length, the result says so. (The recordings came out slightly shorter than the 8 seconds requested because the recorder starts a moment after it is asked to.)
Only the sound is read
Most WebM to MP3 converters decode the whole file. Here only the sound track is read and the video packets are skipped without being decoded, so a long screen recording takes about as long as an audio file of the same length. Each of the 8-second test recordings converted in under a fifth of a second on a laptop. If the WebM has video but no sound track, you're told at once instead of getting a silent MP3.
Why the MP3 can be bigger
Opus is a newer and more efficient codec than MP3. For the same sound, MP3 needs a higher bitrate, so the default here is about a quarter above the original: a browser recording at 128 kbps becomes a 160 kbps MP3. That makes the MP3 a little larger than the sound it came from: our 7-second audio recording went from 114 KB to 142 KB. Speech recorded in mono at a low bitrate grows more, because Opus is especially good at voices. With video in the WebM the MP3 is usually much smaller than the file you started with, because the picture is gone.
| WebM sound | MP3 setting chosen |
|---|---|
| Opus around 128 kbps (browser recordings) | 160 kbps |
| Opus around 160 kbps (downloads) | 224 kbps |
| Opus mono speech | 64 kbps mono |
| Vorbis | matched to its bitrate the same way |
You can override the setting. Choosing far more than the original needs makes a larger file that sounds no better, and the result says so.
No clipping, exact start
A loud recording can come out of MP3 encoding slightly louder than full scale, and players cut those peaks off, which crackles. Each MP3 is decoded again and checked; if it goes over, the volume is lowered just enough and the MP3 is made again. The encoder's short lead-in is marked in the file so the MP3 starts exactly where the recording does.
Private by design
Recordings of meetings and calls are often private. Nothing is uploaded: the WebM is read and the MP3 is written in your browser. To trim the MP3 afterwards use Trim audio; for MP4 and iPhone videos there is MP4 to MP3, and for voice notes in OGG or Opus, OGG to MP3.