How to convert .mkv to .mp4?

This is what I used:

Convert MKV to MP4:

avconv -i input.mkv -codec copy output.mp4

Thanks for your help, everyone.

This is what I used on Ubuntu 14.04 I tried the avconv in the above thread but it didn't work with me. Here is what worked for me.

ffmpeg -i input.mkv -acodec copy -vcodec copy output.mp4

and it shows the following output

ffmpeg version 3.3.3 Copyright (c) 2000-2017 the FFmpeg developers
  built with gcc 4.8 (Ubuntu 4.8.4-2ubuntu1~14.04.3)
  configuration: --extra-libs=-ldl --prefix=/opt/ffmpeg --mandir=/usr/share/man --enable-avresample --disable-debug --enable-nonfree --enable-gpl --enable-version3 --enable-libopencore-amrnb --enable-libopencore-amrwb --disable-decoder=amrnb --disable-decoder=amrwb --enable-libpulse --enable-libfreetype --enable-gnutls --disable-ffserver --enable-libx264 --enable-libx265 --enable-libfdk-aac --enable-libvorbis --enable-libtheora --enable-libmp3lame --enable-libopus --enable-libvpx --enable-libspeex --enable-libass --enable-avisynth --enable-libsoxr --enable-libxvid --enable-libvidstab --enable-libwavpack --enable-nvenc --enable-libzimg
  libavutil      55. 58.100 / 55. 58.100
  libavcodec     57. 89.100 / 57. 89.100
  libavformat    57. 71.100 / 57. 71.100
  libavdevice    57.  6.100 / 57.  6.100
  libavfilter     6. 82.100 /  6. 82.100
  libavresample   3.  5.  0 /  3.  5.  0
  libswscale      4.  6.100 /  4.  6.100
  libswresample   2.  7.100 /  2.  7.100
  libpostproc    54.  5.100 / 54.  5.100
Input #0, matroska,webm, from 'input.mkv':
  Metadata:
    ENCODER         : Lavf57.71.100
  Duration: 00:34:48.80, start: 0.800000, bitrate: 519 kb/s
    Stream #0:0: Video: hevc (Main), yuv420p(tv), 1440x768, SAR 1:1 DAR 15:8, 5 fps, 5 tbr, 1k tbn, 5 tbc (default)
    Metadata:
      DURATION        : 00:34:48.800000000
    Stream #0:1(unknown): Audio: aac (LC), 48000 Hz, stereo, fltp (default)
    Metadata:
      DURATION        : 00:34:48.415000000
File 'output.mp4' already exists. Overwrite ? [y/N] y
Output #0, mp4, to 'output.mp4':
  Metadata:
    encoder         : Lavf57.71.100
    Stream #0:0: Video: hevc (Main) ([35][0][0][0] / 0x0023), yuv420p(tv), 1440x768 [SAR 1:1 DAR 15:8], q=2-31, 5 fps, 5 tbr, 16k tbn, 1k tbc (default)
    Metadata:
      DURATION        : 00:34:48.800000000
    Stream #0:1(unknown): Audio: aac (LC) ([64][0][0][0] / 0x0040), 48000 Hz, stereo, fltp (default)
    Metadata:
      DURATION        : 00:34:48.415000000
Stream mapping:
  Stream #0:0 -> #0:0 (copy)
  Stream #0:1 -> #0:1 (copy)
Press [q] to stop, [?] for help
frame=10439 fps=10024 q=-1.0 Lsize=  132315kB time=00:34:47.59 bitrate= 519.2kbits/s speed=2e+03x      
video:50757kB audio:80879kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.516122%

It works for me and I hope it will work for you.