Convert video to WebM in FFMpeg

What is WebM and why should you use it?

To better understand what WebM is you can watch this short video explanation: What is WebM?. The good news is that as of 2018, ​most browsers support VP9. Read more here. So what is the advantage of the WebM (vp9) codec over MP4 H.264? Basically, libvpx-vp9 can save about 20–50% bitrate compared to libx264.

Simple WebM encoding with FFmpeg

It is really easy to encode video to WebM with FFmpeg.

ffmpeg -i input.mp4 -c:v libvpx-vp9 -crf 45 -b:v 0 output.webm

-crf 45 is the video quality. Lower is better. 0 is lossless. 63 is the worst possible quality. The file size will depend on this.

For better quality, we set the -b:v to 0 which enables “constant quality” (CQ) mode that will ensure that every frame gets the number of bits it deserves to achieve a certain (perceptual) quality level, rather than forcing the stream to have an average bit rate.

Better quality with two-pass encoding

Two-pass encoding means that FFMpeg will run the encoding process twice. The first time it will analyze the input file and make a log file. In the second pass, FFmpeg will use the log file to encode the video. This method takes longer but will create more efficient encodes when a particular target bitrate should be reached.

ffmpeg -y -i original.mkv -c:v libvpx-vp9 -crf 40 -b:v 0 -pass 1 -an -f null NUL && ^
ffmpeg -y -i original.mkv -c:v libvpx-vp9 -crf 40 -b:v 0 -pass 2 crf40.webm

Note that the NUL && ^ is for Windows only. On Linux you can use /dev/null && \

Video examples

Here are some image and video comparisons in case you are wondering which CRF to use.

Original video (14,8 MB)

Two-pass CRF 25 (4,98 MB)

Two-pass CRF 55 (726 kb)

Any good WEBM GUI alternatives?

If you do not like using the command line to convert video there are a bunch of alternatives. I really like webm for retards. It’s a lightweight software that, in addition, to doing the same as above, allows you to set a target file size for the output file.

Although I prefer the command line, it is really nice to avoid the calculation of the bitrate to achieve a specific size. Also, WebMCam is super great for just simple screen capturing to WEBM.


FFmpeg wiki: FFmpeg and VP9 Encoding Guide Werner Robitza: CRF Guide (Constant Rate Factor in x264 and x265)

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