Edit in JSFiddle

$(function(){
    var target_width = $('.target').width(); // get target width
    $('.target2').width( target_width ); // set target width
    $('.target2 .width2 span').text( target_width );
    $('.width_log span').text( $('.target').width() );
    $('.innerWidth_log span').text( $('.target').innerWidth() );
    $('.outerWidth_log span').text( $('.target').outerWidth() );
    
    $('.windowWidth_log span').text( $(window).width() ); // returns width of browser viewport
    $('.documentWidth_log span').text( $(document).width() ); // returns width of HTML document
});

<div class="well target">
  <span class="label label-warning">.target</span>
  <p>set in css:
    <span class="label label-info">width: 400px;</span>
    <span class="label label-info">padding: 10px;</span>
    <span class="label label-info">margin: 12px;</span>
    <span class="label label-info">border: 2px;</span>
  </p>
</div>

<div class="well">
  <p>get .target properties with jQuery:

    <span class="label label-success width_log">
      .target width: <span></span>
    </span>&nbsp;

    <span class="label label-success innerWidth_log">
      .target innerWidth: <span></span>
    </span>&nbsp;

    <span class="label label-success outerWidth_log">
      .target outerWidth: <span></span>
    </span>&nbsp;
  </p>

  <p>
    <span class="label label-success windowWidth_log">
      window width: <span></span>
    </span>&nbsp;

    <span class="label label-success documentWidth_log">
      document width: <span></span>
    </span>&nbsp;
  </p>

</div>

<div class="target2">
  <span class="label label-success width2">width:<span></span></span>
  <span>width of this block set by jQuery (same as .target block)</span>
</div>