When you only need the sound
A video file carries two things: the picture, which takes up almost all of the space, and an audio track that is usually a few percent of the total. If all you care about is what was said or played, the picture is dead weight. Typical cases:
- A recorded lecture, webinar or meeting you want to listen to on a walk or in the car.
- A music performance or rehearsal filmed on a phone, which you want in your music library.
- An interview you plan to edit in an audio editor or send to someone for transcription.
- A voice-over or sound effect you want to reuse in another project.
The size difference is dramatic. A 10-minute 1080p phone clip is often 1 to 1.5 GB, while the same 10 minutes as a 192 kbps MP3 is about 14 MB.
What the converter does to the file
The video stream is dropped entirely and the audio is re-encoded to MP3 with the LAME encoder at the constant bitrate you choose. By default this runs in your browser with FFmpeg compiled to WebAssembly, so the file is read from your disk and the MP3 is written back without leaving the device.
A few details worth knowing:
- One audio track. If the video has several (for example two languages), FFmpeg picks one automatically, normally the track with the most channels. Other tracks are ignored.
- Surround becomes stereo. MP3 supports only mono and stereo, so a 5.1 track from a film is folded down to two channels.
- Sample rate is kept. Phone videos usually carry 48 kHz audio, and the MP3 stays at 48 kHz.
- No sound, no file. A screen recording made without audio, or a clip that was muted, has nothing to extract and the conversion fails.
Accepted inputs are MP4, MOV and WebM, up to 2 GB per file in the browser. You can drop several files and they are converted one after another with the same bitrate.
Choosing the bitrate
The bitrate sets how many bits are spent on each second of audio, and the file size follows directly from it: kbps divided by 8 gives kilobytes per second.
- 128 kbps is about 0.96 MB per minute. Plenty for speech: lectures, calls, podcasts, voice notes.
- 192 kbps is about 1.44 MB per minute. The default, and a safe choice for mixed content and most music.
- 256 kbps is about 1.9 MB per minute. Worth it for music you will hear on good headphones.
- 320 kbps is about 2.4 MB per minute, the highest bitrate MP3 allows.
There is one ceiling to keep in mind: the audio inside the video is already compressed, usually as AAC at 96 to 256 kbps. Converting a 128 kbps AAC track to a 320 kbps MP3 does not bring back anything the first encoder removed; it only makes the file bigger. For speech-heavy video, 128 kbps is genuinely enough. For a concert clip, 192 or 256 kbps keeps the new MP3 from adding audible artifacts on top of the existing ones.
An hour-long lecture is about 58 MB at 128 kbps and 86 MB at 192 kbps.
Large files and slow devices
The browser path has limits: the whole file has to fit into the tab's memory, and a phone decodes far slower than a desktop. Converting a 1.5 GB recording on an older phone may take a long time or fail for lack of memory.
For those cases the page offers server processing. The same FFmpeg command runs natively on our machine. Without an account you get one server run a day for a file up to 500 MB; Premium removes the daily limit and accepts files up to 4 GB. Server files are deleted automatically within about an hour.
After the conversion
Some common next steps:
- Cut the intro, outro or silence with trim MP3 before sharing.
- Need text instead of sound? Send the MP3 to transcription. Uploading a 14 MB audio file instead of a 1 GB video is much faster and gives the same result.
- If the MP3 is still too large for a messenger, re-encode it at a lower bitrate or switch speech to mono.
Text tags from the source, such as a title, are usually carried over, but anything stored as a picture stream, like an embedded thumbnail, is dropped along with the video.