Total width of this box = 324px
(width + margin + padding + borders) 
div.box1 {
width:300px;
margin:20px;
padding:10px;
border:2px solid #666;
background:#009999;
}
Total width of this box = 300px
(width of containing box) 
div.box2 {
width:300px;
margin:20px;
}
		
div.box2 div.padding { 
padding:10px;
border:2px solid #666;
background:#FF0033;
}