html css css-float css-position box-sizing

Not able to arrange two divs side by side

Okay, so I know there are a few questions similar to this on StackOverflow which already have been answered but they didn't help me.

I am building a messaging service and for that I have two divs, contacts_box (300px) and message_box(500px). They are both wrapped inside a parent div which is 800px in width. I want align these two divs side by side inside the parent div. But no matter what I do, I just can't get them to align!

Please take a look at my HTML and CSS and show where I am going wrong with this?

* {
    margin: 0;
    padding: 0;
}
.page_layout {
    position: fixed;
    top: 50px;
    width: 100%;
    height: 100%;
    border: 1px solid green;
}
.page_container {
    width: 800px;
    height: 100%;
    margin: 0 auto;
    clear: both;
    border: 1px solid blue;
}
// Contacts Box and its elements  
.contacts_box {
    float:left;
    height:100%;
    width:300px;
    border:1px dashed magenta;
}
// Message Box and its elements
.message_box {
    float:right;
    height:100%;
    width:500px;
    border:1px dashed lemonchiffon;
}

<html>
  <head>
   <link rel="stylesheet" href="http://kinskeep.com/test.css">
  </head>
<body>
<div class="page_layout">
  <div class="page_container">
    <div class="contacts_box"> CONTACTS BOX </div>
    <div class="message_box">
      <div class="message_displayBox"> Message Box </div>
      <div class="message_textBox"> </div>
    </div>
  </div>
</div>
</body>
</html>

Solution

You can use box-sizing to solve the issue rather than calculating the width and border widths:

Add box-sizing: border-box to the inner containers and box-sizing: content-box to the outer container and there you go!

* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}
.page_layout {
  position: fixed;
  top: 50px;
  width: 100%;
  height: 100%;
  border: 1px solid green;
}
.page_container {
  width: 800px;
  height: 100%;
  margin: 0 auto;
  clear: both;
  border: 1px solid blue;
  box-sizing: content-box;
}
 .contacts_box {
  float: left;
  height: 100%;
  width: 300px;
  border: 1px dashed magenta;
}
 .message_box {
  float: right;
  height: 100%;
  width: 500px;
  border: 1px dashed lemonchiffon;
}

<body>
  <div class="page_layout">
    <div class="page_container">
      <div class="contacts_box">
        CONTACTS BOX
      </div>

      <div class="message_box">
        <div class="message_displayBox">
          Message Box
        </div>

        <div class="message_textBox">
        </div>

      </div>
    </div>
  </div>

</body>