Used if targeting a specific output file size and output quality from frame to frame is of less importance.

This is best explained with an example.

• A video is 10 minutes (600 seconds) long and an output of 200 MiB is desired.
• Since bitrate = file size / duration:

  (200 MiB * 8388.608 [converts MiB to kBit; note: not 8192 as 1 kBit is always 1000 bit]) / 600 seconds = ~2796 kBit/s total bitrate
  2796 - 128 kBit/s (desired audio bitrate) = 2668 kBit/s video bitrate

• The bitrate calculation can be forgo-ed, if the final (average) bitrate wanted is already known.

For two-pass, ffmpeg is run twice, with almost the same settings, except for:

• In pass 1 and 2, use the -x265-params pass=1 and -x265-params pass=2 options, respectively.
• In pass 1, output to a null file descriptor, not an actual file. (This will generate a logfile that ffmpeg needs for the second pass.)
• In pass 1, the audio can be left out by specifying -an.

For libx265, the -pass option (that you would use for libx264) is not applicable.

ffmpeg -y -i input -c:v libx265 -b:v 8000k -x265-params pass=1 -an -f null /dev/null && \
ffmpeg -i input -c:v libx265 -b:v 8000k -x265-params pass=2 -c:a aac -b:a 320k output.mp4