This example consists of a div element with a class of 'box'.
div.box has a width set of 500 pixels. We have added 20 pixels of margin to this div, a 5 pixel border and 20 pixels of padding.
The content area has a width of 500 pixels and we then add on the margin, padding and border to this width. So the total width our div takes up on the page is:
500 + 40 pixels (left and right padding) = 540
540 + 10 pixels (left and right border) = 550
550 + 40 pixels (left and right margin, space around the visible element) = 590 pixels.