Resizing Video for the Web

I often need to resize video for use on the web.

Especially video created by recording my screen with Quicktime on MacOS, which tends to be rather large by default.

Here’s a quick ffmpeg command you can use to resize to something more reasonable.

ffmpeg -i input.mov \
  -vcodec libx264 -crf 23 -preset fast \
  -acodec aac -b:a 128k \
  -movflags +faststart \
  output.mp4

-I input.mov specifies your source video file.

-vcodec libx264 uses the H.264 video codec, which has good we support.

-crf 23 sets a constant rate factor, lower for better quality and a larger file. The range is 0–51 with 23 being a good default.

-preset fast is a good trade-off between encoding time and compression. The options are: ultrafast, superfast, veryfast, faster, fast, medium (default), slow, slower, veryslow. Use slow or slower for better compression/quality at the cost of speed.

-acodec aac uses AAC audio, which is widely supported.

-b:a 128k sets the audio bitrate.

-movflags +faststart optimizes the file for web streaming by moving metadata to the beginning of the file.

Written by Joel Dare on June 6, 2025.

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