Shrink a YouTube video to responsive width

You can make YouTube videos responsive with CSS. Wrap the iframe in a div with the class of "videowrapper" and apply the following styles:

.videowrapper {
    float: none;
    clear: both;
    width: 100%;
    position: relative;
    padding-bottom: 56.25%;
    padding-top: 25px;
    height: 0;
}
.videowrapper iframe {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
}

The .videowrapper div should be inside a responsive element. The padding on the .videowrapper is necessary to keep the video from collapsing. You may have to tweak the numbers depending upon your layout.

If you are using Bootstrap you can also use a responsive embed. This will fully automate making the video(s) responsive.

http://getbootstrap.com/components/#responsive-embed

There's some example code below.

<!-- 16:9 aspect ratio -->
<div class="embed-responsive embed-responsive-16by9">
  <iframe class="embed-responsive-item" src="..."></iframe>
</div>

<!-- 4:3 aspect ratio -->
<div class="embed-responsive embed-responsive-4by3">
  <iframe class="embed-responsive-item" src="..."></iframe>
</div>

Refined Javascript only solution for YouTube and Vimeo using jQuery.

// -- After the document is ready
$(function() {
  // Find all YouTube and Vimeo videos
  var $allVideos = $("iframe[src*='www.youtube.com'], iframe[src*='player.vimeo.com']");

  // Figure out and save aspect ratio for each video
  $allVideos.each(function() {
    $(this)
      .data('aspectRatio', this.height / this.width)
      // and remove the hard coded width/height
      .removeAttr('height')
      .removeAttr('width');
  });

  // When the window is resized
  $(window).resize(function() {
    // Resize all videos according to their own aspect ratio
    $allVideos.each(function() {
      var $el = $(this);
      // Get parent width of this video
      var newWidth = $el.parent().width();
      $el
        .width(newWidth)
        .height(newWidth * $el.data('aspectRatio'));
    });

  // Kick off one resize to fix all videos on page load
  }).resize();
});

Simple to use with only embed:

<iframe width="16" height="9" src="https://www.youtube.com/embed/wH7k5CFp4hI" frameborder="0" allowfullscreen></iframe>

Or with responsive style framework like Bootstrap.

<div class="row">
  <div class="col-sm-6">
    Stroke Awareness
  <div class="col-sm-6>
    <iframe width="16" height="9" src="https://www.youtube.com/embed/wH7k5CFp4hI" frameborder="0" allowfullscreen></iframe>
  </div>
</div>

• Relies on width and height of iframe to preserve aspect ratio
• Can use aspect ratio for width and height (width="16" height="9")
• Waits until document is ready before resizing
• Uses jQuery substring *= selector instead of start of string ^=
• Gets reference width from video iframe parent instead of predefined element
• Javascript solution
• No CSS
• No wrapper needed

Thanks to @Dampas for starting point. https://stackoverflow.com/a/33354009/1011746

I used the CSS in the accepted answer here for my responsive YouTube videos - worked great right up until YouTube updated their system around the start of August 2015. The videos on YouTube are the same dimensions but for whatever reason the CSS in the accepted answer now letterboxes all our videos. Black bands across top and bottom.

I've tickered around with the sizes and settled on getting rid of the top padding and changing the bottom padding to 56.45%. Seems to look good.

.videowrapper {
    position: relative;
    padding-bottom: 56.45%;
    height: 0;
}

@magi182's solution is solid, but it lacks the ability to set a maximum width. I think a maximum width of 640px is necessary because otherwhise the youtube thumbnail looks pixelated.

My solution with two wrappers works like a charm for me:

.videoWrapperOuter {
  max-width:640px; 
  margin-left:auto;
  margin-right:auto;
}
.videoWrapperInner {
  float: none;
  clear: both;
  width: 100%;
  position: relative;
  padding-bottom: 50%;
  padding-top: 25px;
  height: 0;
}
.videoWrapperInner iframe {
  position: absolute;
  top: 0;
  left: 0;
  width: 100%;
  height: 100%;
}
<div class="videoWrapperOuter">
  <div class="videoWrapperInner">
    <iframe src="//www.youtube.com/embed/C6-TWRn0k4I" 
      frameborder="0" allowfullscreen></iframe>
  </div>
</div>

I also set the padding-bottom in the inner wrapper to 50 %, because with @magi182's 56 %, a black bar on top and bottom appeared.