make div's height expand with its content

Solution 1:

You need to force a clear:both before the #main_content div is closed. I would probably move the <br class="clear" />; into the #main_content div and set the CSS to be:

.clear { clear: both; }

Update: This question still gets a fair amount of traffic, so I wanted to update the answer with a modern alternative using a new layout mode in CSS3 called Flexible boxes or Flexbox:

body {
  margin: 0;
}

.flex-container {
  display: flex;
  flex-direction: column;
  min-height: 100vh;
}

header {
  background-color: #3F51B5;
  color: #fff;
}

section.content {
  flex: 1;
}

footer {
  background-color: #FFC107;
  color: #333;
}

<div class="flex-container">
  <header>
    <h1>
     Header   
    </h1>
  </header>

  <section class="content">
    Content
  </section>

  <footer>
    <h4>
      Footer
    </h4>
  </footer>
</div>

Most modern browsers currently support Flexbox and viewport units, but if you have to maintain support for older browsers, make sure to check compatibility for the specific browser version.

Solution 2:

Try this: overflow: auto;

It worked for my problem..

Solution 3:

add the following:

overflow:hidden;
height:1%;

to your main div. Eliminates the need for the extra <br /> for the clear.