How do I convert webm to mp4 for MacOSX?

Get FFmpeg

If you want to use ffmpeg, go and either

• download a recent version of it, or

• install it through Homebrew with

  brew install ffmpeg
  

If you downloaded it manually (not with Homebrew), I would suggest copying the ffmpeg executable file to your PATH, so that you can use it from the Terminal. Let's say you downloaded it to ~/Downloads/ffmpeg/ffmpeg, then do:

sudo mkdir -p /usr/local/bin
sudo cp ~/Downloads/ffmpeg/ffmpeg /usr/local/bin/
sudo chmod +x !$ /usr/local/bin/ffmpeg

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Convert to MP4

Now, by "to MP4", I assume you mean to use H.264 and AAC as video and audio codecs, respectively. For that, the basic command would be:

ffmpeg -i input.webm -c:v libx264 -c:a aac -strict experimental -b:a 192k output.mp4

If you want to control the quality, have a look at the x264 encoding guide. It is set with the -crf option, where the default is 23, and lower means better quality (typical values are from 18 to 28). In the above example it uses the default quality of 23 for video, and 192 kBit/s constant bitrate for audio.

As for audio, the static builds do not support libfdk-aac, but if you have support for it, you should use that instead:

ffmpeg -i input.webm -c:v libx264 -c:a libfdk_aac output.mp4

FDK-AAC gives you better quality than the internal AAC encoder. For controlling audio quality, see the AAC encoding guide.