What you should know
WebM to MP4 is about getting a video out of the browser world. WebM plays very well on the web, but many editing applications, TVs and mobile devices do not recognise it.
The conversion always involves re-encoding, since the codecs differ: VP9 or VP8 must be decoded and then encoded, usually to H.264.
Strengths
- Makes the video readable on devices and software that ignore WebM
- Re-encoding lets you choose a bitrate suited to the destination
- Helps standardise a heterogeneous video library
- Preserves resolution and frame rate
Limitations to know
- Systematic re-encoding, hence loss and computation time
- VP9 decoding is demanding on modest devices
- A long video may hit the tab's memory limit
- Compatibility gain costs size if the bitrate is set poorly
When to use it
Convert when the video must be edited, burned, sent to a living-room device or published on a platform that does not accept WebM. For simple in-browser playback, keep the WebM: it is often lighter.
Quality, size and compatibility
Like any re-encode, quality depends on the output bitrate. Starting from an already compressed WebM, aim for a bitrate that does not fall below the source: you would only gain artefacts.
Frequently asked questions
- Can WebM be converted without re-encoding?
- No, WebM codecs (VP8, VP9) are not compatible with mainstream containers at the same level. Decoding and H.264 encoding are required.
- Why is the audio kept while the video is re-encoded?
- Opus audio is generally converted to AAC along the way, to stay compatible with the MP4 container and common players.