Where WebM recordings come from
If you recorded something inside a web page, there is a good chance the file is WebM. Chrome, Edge and Firefox expose a recording interface called MediaRecorder to web apps, and in those browsers it produces WebM. Online voice recorders, browser-based screen recorders, interview and dictation tools, and the record button in many web apps all rely on it. Safari is the exception: its MediaRecorder writes M4A, which the M4A to MP3 page handles.
WebM also arrives from outside the browser. Screen recorders on Linux often save it by default, and video downloaded from the web is frequently WebM as well. The file may contain a full screen recording with video, or only audio if it came from a microphone recorder.
Opus inside WebM, and what the converter does with it
WebM is a container built on Matroska. Its video track, when there is one, is usually VP8 or VP9, and its audio track is Opus or, in older files, Vorbis. Browser recordings almost always use Opus, which keeps speech clear at small file sizes and is why a long recording from a web app can be surprisingly small.
The converter decodes the Opus or Vorbis audio and encodes it as MP3 with the LAME encoder at a constant bitrate. If the file has a video track, it is discarded. If the file is audio only, which is what most microphone recorders produce, the whole thing is simply re-encoded. The output keeps the original name with .mp3 on the end.
One known quirk of MediaRecorder files is worth mentioning. They are written as a stream while the recording is still going, so the finished file often has no duration in its header. Players show 0:00, and seeking does not work. The converter reads the stream through to the end, and the MP3 it writes has a normal length that any player can seek through.
Apps and devices that refuse WebM
WebM was designed for the web and that is where it is best supported. Outside the browser it is often turned away. The built-in players on iPhone and Mac do not open it, many audio editors and DAWs will not import it, and upload forms on course platforms, podcast hosts and some transcription services list MP3, WAV and M4A but not WebM. Car stereos and portable players never had it.
MP3 is accepted nearly everywhere, which is why converting once is the usual fix. If the next step is editing rather than sharing, an uncompressed WAV is the better target, and the WAV pages on this site convert in the same way.
Picking a bitrate
The converter offers 128, 192 or 320 kbps, with 192 as the default. Opus in a browser recording is already lossy, and MP3 is lossy too, so the conversion loses a little more. The bitrate decides how little.
- 128 kbps: plenty for a microphone recording of one or more people talking. Speech is fine between 64 and 128 kbps, and this setting gives the smallest file at about 1 MB per minute.
- 192 kbps: the default, and a sensible choice for a screen recording that mixes a voice with system audio or music. About 1.4 MB per minute.
- 320 kbps: only worth it when the recording carries music you care about. It cannot restore detail the Opus encoder removed, so for a voice recording it just makes a larger file. About 2.4 MB per minute.
Nothing is uploaded
The converter is FFmpeg compiled to WebAssembly, running in your browser. Your WebM stays on your device, and nothing is stored on our side. On first use the browser downloads the conversion engine, about 32 MB, and caches it; after that it also works offline. One file at a time, up to a practical limit of about 2 GB on a computer, less on a phone or tablet because of browser memory limits.
The finished MP3 lives only in the open tab, so press Download before you reload or close the page. There is no sign-up or watermark, and you can convert as many files as you like. Only local files are accepted; a link to a page that hosts the recording will not work. It runs in current Chrome, Edge and Firefox, and in Safari 16 or newer.
If you actually need the words
Browser recordings are often interviews, meetings or dictation, and the real goal is a transcript. After the conversion, a Transcribe this file button sends the MP3 into the transcription box further down this page. That part runs on our servers, so the file is uploaded for it. You get a timestamped, editable transcript in 90+ languages and can export it as TXT, DOCX, PDF, SRT, VTT or CSV. It is free to try without an account. The WebM to text page accepts the original file directly if you do not need the MP3 at all.