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WebM to MP4
Convert WebM to MP4 Online — Free & Instant
Turn WebM screen recordings into universally compatible MP4 files.
Nothing is enforced, but the whole recording is read into one buffer to decode its audio to 32-bit float, and the MP4 is built up in another beside it — so files much past ~100 MB risk exhausting the tab.
About WebM to MP4
WebM is the default format for browser screen recordings and many web-based recording tools, but it has limited compatibility outside the web. Many devices, social media platforms (Instagram, TikTok, LinkedIn), video editors (iMovie, Premiere Pro, DaVinci Resolve), and messaging apps do not handle WebM well or at all. This tool transcodes your WebM file to H.264/AAC MP4 — the most widely supported video format across virtually every device, platform, and application. The H.264 codec provides excellent compression while maintaining sharp video quality, and AAC audio ensures the soundtrack comes through cleanly. Note that this is a genuine re-encode rather than a container swap: the WebM's VP8/VP9 video is decoded and re-encoded as H.264, so expect a small quality cost. Encoding uses the browser's built-in WebCodecs API with mp4-muxer — hardware-accelerated where your device offers it — and runs entirely in the tab, so no video files are uploaded to any server. In-browser encoding requires Chrome or Edge. This makes it safe for converting screen recordings of confidential work, product demos, internal presentations, and any other sensitive content. Common use cases include converting screen recordings for upload to social media, preparing tutorial videos for platforms that require MP4, and making recordings compatible with video editing software for post-production.
Questions
Why convert WebM to MP4?
WebM is great for web playback but has limited support on iOS, some Android apps, and many video editors. MP4 (H.264/AAC) is the most universally compatible video format.
Will the quality change?
Yes — the video is fully re-encoded from VP8/VP9 to H.264, so this is not a lossless remux. The target is 5 Mbps, high enough that the difference is hard to see on typical screen recordings. The output may be larger or smaller than the original depending on the content.
How long does conversion take?
It depends on the video length and your device. Encoding runs through the browser's built-in WebCodecs API, which uses your hardware encoder where one is available, so a 1-minute clip is usually well under a minute. This needs Chrome or Edge — Firefox and older Safari don't expose the encoder.
Can I convert a 4K WebM?
Yes, but the output is capped at 1920 pixels on the longest side, so a 3840×2160 recording comes out at 1920×1080. Anything at 1080p or below keeps its original dimensions, rounded to even numbers as H.264 requires.
What happens to the audio track?
The WebM's Opus or Vorbis audio is decoded and re-encoded to AAC at 128 kbps — the codec MP4 players expect. A recording with no audio track converts fine and produces a video-only MP4.
Is my file uploaded to a server?
No. Transmute processes everything locally in your browser using JavaScript and WebAssembly. Your files never leave your device — there is no server, no upload, no cloud processing.