Convert a video to WebM
The video to WebM tool re-encodes an MP4, MOV, MKV, or AVI as a WebM file with VP9 video and Opus audio. VP9 and Opus are open, royalty-free codecs, and WebM is the container browsers were built to play natively. ffmpeg compiled to WebAssembly does the work inside the browser tab, with the same encoders as desktop ffmpeg, and the video is never uploaded.
Why WebM
WebM files are usually smaller than an H.264 MP4 at the same visible quality, because VP9 is a newer and more efficient codec. Chrome, Firefox, and Edge play VP9 WebM, and Safari supports it on recent versions, which makes it a good choice for video that will be embedded on a web page. It is also the native format for screen recordings made by the browser's own media recorder, and some wikis and forums accept only WebM uploads.
VP9 encoding takes longer than x264 for the same clip. Keep the tab open until the download appears.
The CRF option
CRF runs from 0 to 63 for VP9 and defaults to 30. Lower numbers mean higher quality and bigger files. The scale is wider than x264's, so the numbers do not map one to one with MP4 settings.
30 is a good balance for web delivery: close to the source on a phone or laptop screen, with a file noticeably smaller than the input. For higher quality, use 24 to 27. For a small file where some softness is acceptable, such as a background loop or a preview, try 35 to 40. Values past 45 show visible blurring in detailed scenes.
Lower numbers both increase the file size and slow the encode, so pick the highest CRF whose output still looks right on the screen it is meant for.
Audio
Audio is encoded with Opus, which handles speech and music well at modest bitrates. If the source has no audio track, the output has none either.
If what you want is the sound by itself, video to MP3 pulls it out as an MP3.
Resolution and naming
This tool does not scale the picture. A 1080p source comes out 1080p. If you need both a smaller frame and a smaller file, compress video lets you set a maximum height, but it writes MP4 rather than WebM.
The output keeps the base name and swaps the extension, so "clip.mp4" becomes "clip.webm". Dropping a WebM re-encodes it with the chosen CRF and adds "-converted" to the name so the original is not overwritten.
When MP4 is the better output
WebM is a web format first. Many desktop editors, built-in phone players, and older TVs handle MP4 but not WebM. If the file is going to be attached to an email, imported into an editing program, or played on a device you do not control, choose video to MP4 instead. For a short silent loop that has to be an image file, video to GIF is the right one.
Frequently asked questions
What codecs does the WebM use?
VP9 for video and Opus for audio, which is the standard pairing for WebM and what browsers expect.
What CRF should I use?
The default of 30 suits web delivery. Use 24 to 27 for higher quality and 35 to 40 for a smaller file where some softness is fine.
Why does the CRF range go to 63?
VP9 uses a wider scale than x264, so its numbers do not map one to one with MP4 settings. 30 on VP9 is roughly comparable to the MP4 default of 23.
Will the WebM play on my phone?
In a browser, yes. Some built-in video players and older TVs do not support WebM, so choose MP4 for files that go outside the web.
Does the resolution change?
No. The frame size stays as it was. The compress video tool can cap the height, but it writes MP4.
Which input formats are accepted?
MP4, MOV, WebM, MKV, and AVI.