CSS box-sizing Property

Include padding and border in the element's total width and height:

#example1 {
box-sizing: border-box;
}

Definition and Usage

The box-sizing property defines how the width and height of an element are calculated: should they include padding and borders, or not.

Default value:	content-box
Inherited:	no
Animatable:	no.
Version:	CSS3
JavaScript syntax:	object.style.boxSizing="border-box"

Inherited : "Inherited = no" means that it cannot takes (inherit) it's value from it's parent element.
object - object in javascript means the element on which box-sizing is applied.
Animatable - "Animatable = no" means that it cannot be animated with CSS @keyframes.

The numbers in the table specify the first browser version that fully supports the property.

Numbers followed by -webkit- or -moz- specify the first version that worked with a prefix.

Property
box-sizing	10.0 4.0 -webkit-	8.0	29.0 2.0 -moz-	5.1 3.2 -webkit-	9.5

box-sizing: content-box|border-box|initial|inherit;

Value	Description	Demo
content-box	Default. The width and height properties (and min/max properties) includes only the content. Border and padding are not included
border-box	The width and height properties (and min/max properties) includes content, padding and border
initial	Sets this property to its default value.
inherit	Inherits this property from its parent element.

Specify two bordered boxes side by side:

div {
  box-sizing: border-box;
  width: 50%;
  border: 5px solid blue;
float: left;
}

Set the "universal box-sizing":

* {
box-sizing: border-box;
}