Convert video to WebM

Worked example

VP9 compresses noticeably better than H.264 at the same visual quality, and encodes a great deal more slowly.

The equivalent command

ffmpeg -i holiday-clip.mp4 -c:v libvpx-vp9 -crf 33 -b:v 0 -pix_fmt yuv420p -deadline realtime -cpu-used 5 -c:a libopus -b:a 160k -threads 1 -y out-holiday-clip.webm

This is exactly what runs in the browser. Copy it to run the same job locally on a file too large for a tab.

What to expect

Size is only predictable when you set a bitrate. These figures are for the bitrate example above:

At a constant quality the encoder lets the bitrate follow the content, so the output size genuinely cannot be predicted in advance — a slide deck and handheld footage of leaves differ by an order of magnitude at the same setting.

Why it matters

VP9 needs one setting that catches almost everyone: a constant-quality encode must pass a zero video bitrate alongside the quality value, or the quality setting acts as a ceiling rather than a target and the encoder sits far below what you asked for. That is the usual reason a WebM looks worse than the MP4 made with "the same" number — and the CRF scales are not comparable either, since VP9 runs to 63 where H.264 stops at 51.