WebM to MP4 converter
This WebM to MP4 converter makes recordings from the browser play on iPhones, in Windows Media Player and in editing software. The picture is re-encoded as H.264 and the sound as AAC – all in your browser, with no upload.
Source: MDN – Media container formats: WebM and MP4. Updated: .
How it is calculated
Why convert WebM to MP4?
WebM is the open video format of the web: it usually holds VP8, VP9 or AV1 picture with Opus or Vorbis sound. Browser screen recordings, many downloaded web videos and captures from Chrome or Firefox come as WebM. Modern browsers and Android play it; the iPhone photo app, older Windows players, TVs and many editors often do not. MP4 with H.264 picture and AAC sound plays practically everywhere.
What the tool does
Because the codec changes, repackaging alone is not enough: the picture is recalculated as H.264 by your browser’s video encoder (WebCodecs) and the sound as AAC. Depending on your device that takes roughly as long as the video itself; at the usual bitrate the quality stays very close to the original, though a small loss is unavoidable whenever video is re-encoded. If your WebM already contains H.264 (rare), repackaging is enough.
| Setting | What happens | When it makes sense |
|---|---|---|
| Re-encode as H.264 (default) | Picture becomes H.264, sound becomes AAC | For maximum compatibility: iPhone, TV, Windows, editing |
| Copy unchanged | Picture and sound stay, only the container changes | Fast and lossless when the target device understands VP9 and Opus in MP4 |
Limits
- The result is built in memory; files up to 4 GB are allowed, but very long recordings need a device with plenty of RAM.
- If your browser cannot create H.264 (some Linux browsers), the tool says so; Chrome, Edge and Safari can.
- Screen recordings with a varying frame rate keep their timing.
Privacy
The video is only read piece by piece from your device; there is no upload. The open-source library Mediabunny (MPL-2.0) reads and writes the containers. For further steps: Trim video, Compress video or just the sound with Video to MP3.
Frequently asked questions
How do I convert WebM to MP4?
Choose the WebM file and click “Convert WebM to MP4”. The picture is re-encoded as H.264 and the sound as AAC, then the MP4 is ready to download. Nothing is uploaded.
Why does my WebM video not play on an iPhone?
iPhones and many older devices do not reliably support VP9 and Opus. As an MP4 with H.264 and AAC the video plays there.
Is quality lost when converting WebM to MP4?
Re-encoding always costs a little quality, but at a normal bitrate it is hardly visible. For lossless repackaging choose “Copy unchanged” – the MP4 then only plays on devices that understand VP9 and Opus.
How long does the conversion take?
Re-encoding takes roughly as long as the video, depending on your device. A computer with hardware acceleration is much faster.
Can I convert a screen recording from the browser?
Yes. Browsers usually save screen recordings as WebM; after converting, the recording also plays in editing software and on phones.
Is my video uploaded?
No. The video is read and converted in your browser and never leaves your device.
Sources and legal basis
- MDN – Media container formats: WebM and MP4
- WebM Project – About WebM (VP8/VP9/AV1 with Vorbis/Opus)
- MDN – Web video codec guide: H.264 (AVC), VP9, AV1 and their browser support
- MDN – VideoEncoder.configure(): bitrate in Bit pro Sekunde
- MDN – EncodedVideoChunk.type (key/delta)
- ITU-T H.264 – Advanced video coding for generic audiovisual services
- Mediabunny 1.61.0 – Vanilagy (MPL-2.0), Quelltext
- Licenses of the media tools (MPL-2.0)
As of: · Editorial policy
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