ffmpeg -i crf21.mkv -c copy crf21.mp4
ffmpeg version N-76224-ga7c5005 Copyright (c) 2000-2015 the FFmpeg developers
  built with gcc 5.2.0 (GCC)
  configuration: --enable-gpl --enable-version3 --disable-w32threads --enable-avisynth --enable-bzlib --enable-fontconfig --enable-frei0r --enable-gnutls --enable-iconv --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libdcadec --enable-libfreetype --enable-libgme --enable-libgsm --enable-libilbc --enable-libmodplug --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libopus --enable-librtmp --enable-libschroedinger --enable-libsoxr --enable-libspeex --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvo-aacenc --enable-libvo-amrwbenc --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxavs --enable-libxvid --enable-lzma --enable-decklink --enable-zlib
  libavutil      55.  4.100 / 55.  4.100
  libavcodec     57.  9.100 / 57.  9.100
  libavformat    57. 10.101 / 57. 10.101
  libavdevice    57.  0.100 / 57.  0.100
  libavfilter     6. 12.100 /  6. 12.100
  libswscale      4.  0.100 /  4.  0.100
  libswresample   2.  0.100 /  2.  0.100
  libpostproc    54.  0.100 / 54.  0.100
Input #0, matroska,webm, from 'crf21.mkv':
  Metadata:
    COMPATIBLE_BRANDS: isomiso2avc1mp41
    MAJOR_BRAND     : isom
    MINOR_VERSION   : 512
    ENCODER         : Lavf57.10.101
  Duration: 00:01:10.02, start: 0.000000, bitrate: 13736 kb/s
    Stream #0:0: Video: h264 (High), yuv420p, 1920x1080 [SAR 1:1 DAR 16:9], 60 fps, 60 tbr, 1k tbn, 120 tbc (default)
    Metadata:
      CREATION_TIME   : 2016-03-04 03:04:46
      HANDLER_NAME    : Video Media Handler
      ENCODER         : Lavc57.9.100 libx264
      DURATION        : 00:01:10.011000000
    Stream #0:1(eng): Audio: aac (LC), 48000 Hz, stereo, fltp (default)
    Metadata:
      CREATION_TIME   : 2016-03-04 03:04:46
      HANDLER_NAME    : Sound Media Handler
      DURATION        : 00:01:10.016000000
[mp4 @ 000001b7ef6f9920] Codec for stream 0 does not use global headers but container format requires global headers
[mp4 @ 000001b7ef6f9920] Codec for stream 1 does not use global headers but container format requires global headers
Output #0, mp4, to 'crf21.mp4':
  Metadata:
    COMPATIBLE_BRANDS: isomiso2avc1mp41
    MAJOR_BRAND     : isom
    MINOR_VERSION   : 512
    encoder         : Lavf57.10.101
    Stream #0:0: Video: h264 ([33][0][0][0] / 0x0021), yuv420p, 1920x1080 [SAR 1:1 DAR 16:9], q=2-31, 60 fps, 60 tbr, 16k tbn, 1k tbc (default)
    Metadata:
      CREATION_TIME   : 2016-03-04 03:04:46
      HANDLER_NAME    : Video Media Handler
      ENCODER         : Lavc57.9.100 libx264
      DURATION        : 00:01:10.011000000
    Stream #0:1(eng): Audio: aac ([64][0][0][0] / 0x0040), 48000 Hz, stereo (default)
    Metadata:
      CREATION_TIME   : 2016-03-04 03:04:46
      HANDLER_NAME    : Sound Media Handler
      DURATION        : 00:01:10.016000000
Stream mapping:
  Stream #0:0 -> #0:0 (copy)
  Stream #0:1 -> #0:1 (copy)
Press [q] to stop, [?] for help
frame= 4200 fps=0.0 q=-1.0 Lsize=  117481kB time=00:01:10.01 bitrate=13745.5kbits/s
video:115643kB audio:1706kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.112805%