Each HTML element is represented as a rectangular box, it consists of margins, borders, padding, and the actual content. The default CSS box model used to calculate widths and heights of elements, the width/height of an element gives the width/height of the content of the box, excluding padding and border.

and they are calculated like this:

Width = width + padding-left + padding-right + border-left + border-right

Height = height + padding-top + padding-bottom + border-top + border-bottom

so if we take this as an example :

.box { width: 300px; padding: 10px; border: 5px solid #e9573f; margin: 10px; }

the width of the rendred box will be calculated like this :

300px (width) + 20px (left + right padding) + 10px (left + right border) + 20px (left + right margin) = 350px

which will give us a box with 350px width, so in this case you have to change the width of the box to 270px, and this is where the box-sizing property comes to the rescue.

box-sizing allows you to switch box models :

content-box: This is the default style as specified by the W3C. The width and height properties are measured including only the content, but not the border, margin, or padding.

border-box: The width and height properties include the padding and border, but not the margin. This is the box model used by IE when the document is in Quirks mode.

padding-box : The width and height properties include the padding size, and do not include the border or margin (only in Firefox).

To save you time, you can apply it using the universal selector

* { -moz-box-sizing: border-box; -webkit-box-sizing: border-box; box-sizing: border-box; }

box-sizing is pretty well supported but partially and i mean that only Firefox support the three models, the others browsers only support content-box and border-box since the padding-box value has been added to the spec very recently.