MP4 to WebM
VP8 video with Opus audio, in a patent-free container.
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Why WebM
WebM is the open, royalty-free video format. If you are shipping video inside a product, or serving it from a page where you would rather not think about h.264 licensing, WebM is the format that avoids the question entirely. Every current browser plays it, including Safari from version 14 onward.
VP8, not VP9
Two codecs can live in a WebM file. VP9 compresses better, but its encoder crashes this WebAssembly build of FFmpeg a frame or two in, a known limitation of running libvpx without thread support. This tool uses VP8, which is stable and much faster to encode. The result is a valid WebM that plays anywhere WebM plays; it is simply larger than a VP9 file would be.
If file size is what you care about most, h.264 in MP4 will beat VP8 comfortably. Choose WebM when the open format is the point.
Opus is the part nobody mentions
The audio in a WebM here is Opus, and it is the most straightforwardly excellent codec in the whole build. At 96 kbps it is transparent for music where MP3 needs half again as much, and at 32 kbps it carries speech cleanly, which is a bitrate where AAC is already struggling. If the video is mostly someone talking, the audio side of a WebM conversion is a genuine upgrade over the MP4 you started with, even as the picture side gives ground.
Expect it to take a while
libvpx is slow in WebAssembly, several times slower than x264 on the same footage, and this build runs on one thread. A minute of 1080p that would convert to MP4 in under a minute can take considerably longer here. Two things help, both worth doing before you start rather than after: drop the resolution to 720p with resize if the destination does not need more, and trim to the section you actually want with cut. Encoding is charged by the pixel and by the second, and both of those cut the bill directly.
Where WebM is the right answer
- Video on a page you control. Serve WebM with an MP4 fallback and the licensing question never arises for the majority of your traffic.
- Anything bundled into open source software, where shipping an h.264 decoder is a legal complication rather than a technical one.
- Wikimedia and similar archives, which will not accept patent-encumbered formats at all.
Where it is the wrong answer is anything leaving your control: TVs, older phones, PowerPoint, most video editors and a number of social platforms will refuse it. That is what the conversion back is for.
Common questions
Is WebM better than MP4?
It is free of patent licensing, which is its main advantage. On file size, the VP8 encoding used here is not as efficient as h.264, so an MP4 will usually be smaller. MP4 also wins on universal playback, TVs, editors and older phones all take it without question.
Can I get VP9 instead?
Not in the browser. The VP9 encoder is not stable in this WebAssembly build, so offering it would just mean failed jobs. Reading VP9 files works fine, so converting the other way is not a problem.
Why is this slower than converting to MP4?
libvpx is slower than x264 in WebAssembly. Lower the resolution first if the file is long.