I just did:

ffmpeg -i inputfile -r 30 -vcodec libx264 -an -s hd480 output_30.mp4


ffmpeg -i inputfile -vcodec libx264 -an -s hd480 output_60.mp4

on a 60fps file.

Why is the second file 27 MB and the first file 49 MB despite the fact that the first file contains only every second frame?

  • 1
    Please show the complete console output from each command.
    – llogan
    Jul 27, 2015 at 3:14

1 Answer 1


Did you actually get every second frame? My impression is that you got every frame, but you halved the framerate. Therefore, the length of the video in seconds is twice as long for the first compared to the second, and assuming equal rate controls (e.g. 1 megabit per second), the twice-as-long-file will thus be approximately twice as big, which is what you got.

  • That may be the case. What would be the correct command.
    – student
    Nov 3, 2015 at 18:34
  • -v:b 500 for the first, and -v:b 1000 for the second. If my hypothesis is correct, using double bitrate for the second should give exactly identical quality for both videos (i.e. they should look visually indistinguishable, other than them being twice as slow/fast). Nov 4, 2015 at 1:17

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