WebM to MP4 Converter
Your files never leave your device
Conversion runs inside your browser using WebAssembly. Nothing is uploaded; the only downloads are this page and the conversion engine. You can confirm it in your browser's network tab.
Why convert WebM to MP4?
Screen, webcam and call recordings made in a browser are WebM, which many editors (including iMovie), TVs and messaging apps won’t accept.
What changes in the MP4
- VP8/VP9/AV1 video is re-encoded to H.264 and Opus/Vorbis audio to AAC.
- File size can go up or down: browser recordings are encoded in real time at a fixed bitrate, while the converter aims for constant quality.
- Transparency in a WebM (VP8/VP9 with alpha) is lost; H.264 MP4 can’t store it.
How Convertz converts it
FFmpeg (WebAssembly) re-encodes the video to H.264 at CRF 23 (visually close to the source at a sensible size) with AAC audio at 160 kbps, and moves the index to the start of the file so it streams on the web. It runs on a single CPU core in your browser, so long recordings take a while.
Everything happens on your device; the file is never uploaded.
Common pitfalls
- Files over about 2 GB won’t fit in browser memory.
WebM vs MP4
| Property | WebM | MP4 |
|---|---|---|
| Full name | WebM (Google, 2010) | MPEG-4 Part 14 |
| Compression | Lossy (VP8, VP9 or AV1) | Depends on codec (usually H.264) |
| Transparency | Supported with VP8/VP9 | No (in practice) |
| Works with | Chrome, Firefox and Edge; Safari support depends on the version and codec. Many video editors, TVs and messaging apps don’t accept it. | Essentially everything, when the video is H.264 with AAC audio. |
What is WebM?
An open, royalty-free video format based on Matroska, holding VP8, VP9 or AV1 video with Opus or Vorbis audio. Chrome and Firefox produce WebM when they record the screen, camera or a call, and many downloaded web videos are WebM.
What is MP4?
The most widely supported video container. MP4 files usually hold H.264 or HEVC video with AAC audio, and play on virtually every phone, browser, TV and editing app.
Related conversions
The converter also handles 22 input formats in total. See all formats