Keep a Box dimension while using padding

How to use padding inside of a box without altering its size?

The box-sizing CSS3 property can do just this. The border-box value (as opposed to the content-box default) makes the final rendered box the declared width, and any border and padding cut inside the box. We can now safely declare our textarea to be of 100% width, including pixel-based padding and border, and accomplish out layout perfectly.

View Demo

CSS:

-webkit-box-sizing: border-box; /* Safari/Chrome, other WebKit */
	-moz-box-sizing: border-box;    /* Firefox, other Gecko */
	box-sizing: border-box;         /* Opera/IE 8+ */

SOURCE