If you have ever been unsure whether to save a video as MP4 or WebM, you are not alone. They look identical when they play, but underneath they make different trade-offs. Here is a plain-language guide to choosing, and to converting between them privately in your browser.
What MP4 actually is
MP4 is a container — a wrapper — that almost always holds H.264 video and AAC audio. Its defining feature is ubiquity: practically every phone, TV, editor, browser, and social platform accepts an H.264 MP4. That universality makes it the safe default whenever a file needs to “just work” somewhere else, especially if it is going into another piece of software.
What WebM brings
WebM is the web-native, royalty-free container, typically holding VP9 (or newer AV1) video with Opus audio. At the same visual quality it is often noticeably smaller than H.264, and being open and free of licensing fees is exactly why browsers and many websites prefer it for embedded playback.
A simple decision rule
Ask one question: where will this video live? If the answer is “on a web page,” or “I want the smallest file at this quality,” WebM is an excellent choice. If the answer is “it needs to play on anything” or “I’m importing it into an editor or sending it to someone who might be on older software,” choose MP4. When in doubt, MP4 is the more conservative pick.
Converting without uploading
VideoDock’s in-browser re-encoder writes WebM, because that is what the MediaRecorder API can reliably produce on every device. The optional FFmpeg engine — active when the app is served with cross-origin isolation — adds true MP4 muxing, so you can convert in either direction. Crucially, both paths run entirely on your machine; the file is never uploaded for conversion.
A note on “converting” quality
Converting between formats means re-encoding, which is lossy. Going MP4 → WebM → MP4 repeatedly will slowly degrade quality, just like photocopying a photocopy. Convert from your highest-quality source whenever you can, and keep that source around. If you only need broader compatibility and not a specific container, check first whether your destination already accepts WebM — increasingly, it does, and then you can skip the conversion entirely.
Re-encode to WebM (MP4 with the FFmpeg build).