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WebM
What is a WebM file?
The royalty-free web video format. Small files, plays in browsers, ignored by most TVs.
What WebM is
WebM is a container: a wrapper that holds streams encoded by something else. It is used for the web and streaming.
The extension is .webm, and the full name is WebM Video. Both matter less than what the file can hold, which is what the rest of this page is about.
Where WebM came from
Google published it in 2010.
Age is worth knowing here for one practical reason: the older a format is, the more programs have had time to learn it.
The specification is public
Anyone can implement it without asking permission or paying a licence, which is why it turns up in so many programs and why files written twenty years ago still open.
It can be transparent
WebM carries an alpha channel, so a logo keeps its soft edges over whatever sits behind it instead of arriving in a white box.
WebM is a box, and the codec is what is in it
It usually holds VP8, VP9, AV1, Opus and Vorbis. This is the distinction behind most "why will this not play" questions: two files with the same extension can contain entirely different encodings, and a player that refuses one may be perfectly happy with the other.
It also explains why changing container is often fast and lossless while changing codec is neither. Moving the same stream into a different box rewrites the box; re-encoding rewrites the picture.
What else it can carry
WebM can hold several audio tracks, subtitle tracks and a layout that lets it start playing before it has finished arriving.
That matters mostly when converting: whatever the target cannot hold is dropped, usually without a warning.
What opens WebM
VLC and FFmpeg read it, and so do most programs of the same kind.
If a file will not open, the format is rarely the problem — it is more often that the program predates it. Converting to something older is the reliable way past that, and it is what the rest of this site is for.
Opening it in a browser
Every current browser reads it.
That makes it a safe thing to put on a page or attach to a message without wondering what the other end has installed.
It is a delivery format
WebM is meant to be handed over rather than worked in. Editing one is possible and rarely pleasant; the sane approach is to change the source and export again.
What goes wrong with it
The recurring complaints: support outside its home ground is uneven.
None of these is a reason to avoid the format. They are the things worth knowing before you are surprised by one, which is a different claim and a more useful one.
The facts, in one place
| Extension | .webm |
|---|---|
| Media type | video/webm |
| Published by | |
| First published | 2010 |