The element will not be composed of the (width value) + (padding value) +(border width value) but rather the value declared for css width property itself.

   -moz-box-sizing:    border-box;
   -webkit-box-sizing: border-box;
    box-sizing:        border-box;

http://www.soyoucode.com/2012/quick-tip-draw-border-element-css

Pro and cons of the use of border-box

http://www.smashingmagazine.com/2012/06/14/coding-qa-with-chris-coyier-box-sizing-and-css-sprites/