title: How to convert 4:3 video to 16:9 without stretching the original in ffmpeg
body: <p>I would like to convert some 4:3 videos that I took on an iPhone to 16:9, by adding black bars to both sides instead of stretching the video.</p>
  
  <p>I tried something like this without success:</p>
  
  <p><code>ffmpeg -y -i input.mov -aspect 1.7777777777777777 -s 640x360 output.mp4
  </code></p>
  
  <p>Even though the output has the correct size, the video is stretched, which is not what I want.</p>
  
  <p>How can I keep the proportion between the width and height of the original video, and add extra black bars to both sides to avoid stretching?</p>
  
  <p>This would be similar to how YouTube does it when uploading a video from an iPhone directly to YouTube.</p>
  
  <p>Thanks!</p>