Convert WebM to MP4

WebM files from screen recorders and browsers do not open in every player or upload form. This page opens the video converter with MP4 selected and re-encodes the WebM to H.264 video with AAC sound.

Runs on your device

More options in the full Video format converter.

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How to convert WebM to MP4

Choose one or more WebM files.
Keep Format on MP4; optionally set a maximum resolution.
Select Convert to MP4.
Download each MP4, or all of them as a ZIP.

Worked example

In the converter's automated test, a 2-second 320×180 WebM with VP9 video and Opus sound becomes an MP4 with H.264 video (48 frames, 320×180) and AAC audio, and the file list notes both as re-encoded.

Formats and limits

Input	WebM
Output	MP4
Limits	Input files up to 4 GB are read in pieces; the converted file is built in memory, so outputs above about 2 GB may fail. Needs a browser with WebCodecs (current Chrome, Edge, Safari 17+ or Firefox 130+). Other browsers are told so before any work starts.
Engine	Mediabunny (WebCodecs): streams are copied when compatible, otherwise re-encoded; MP3 via a WebAssembly encoder

Limitations

Re-encoding changes the picture slightly; it cannot add detail the WebM did not have.
If this browser cannot encode H.264, another codec MP4 can hold (H.265, AV1 or VP9) is used and the file list names it.
The MP4 is built in memory, so outputs above about 2 GB may fail.
Needs a browser with WebCodecs: current Chrome or Edge, Safari 17 or later, or Firefox 130 or later. Other browsers are told so before any work starts.

Questions

Why does WebM to MP4 take longer than other conversions?

WebM holds VP8, VP9 or AV1 video, which MP4 players rarely expect, so the video is re-encoded instead of copied. Re-encoding runs on your device and takes longer for long or high-resolution videos.

Can I make the MP4 smaller at the same time?

Yes. Set Maximum resolution to 1080p, 720p or 480p. Videos are never enlarged.

Is it safe to convert WebM to MP4 online?

On this page nothing is uploaded: the WebM is read, re-encoded and saved as MP4 inside your browser.

Privacy

Runs on your device. Files and text are processed in this browser tab and are not uploaded.

This page opens the Video format converter ready for this task. See the privacy policy for how toolsdocks handles data.