This is an experimental technology
Because this technology's specification has not stabilized, check the compatibility table for usage in various browsers. Also note that the syntax and behavior of an experimental technology is subject to change in future versions of browsers as the specification changes.

Summary

The box-sizing property is used to alter the default CSS box model used to calculate width and height of the elements. It is possible to use this property to emulate the behavior of browsers that do not correctly support the CSS box model specification.

Initial valuecontent-box
Applies toall elements that accept width or height
Inheritedno
Mediavisual
Computed valueas specified
Animatableno
Canonical orderthe unique non-ambiguous order defined by the formal grammar

Syntax

/* Keyword values */
box-sizing: content-box;
box-sizing: border-box;

/* Global values */
box-sizing: inherit;
box-sizing: initial;
box-sizing: unset;

Values

content-box
This is the initial and default value as specified by the CSS standard. The width and height properties are measured including only the content, but not the padding, border or margin. Note: Padding, border & margin will be outside of the box e.g. IF .box {width: 350px;} THEN you apply {border: 10px solid black;} RESULT {rendered in the browser} .box {width: 370px;}
So simply the dimension of element is calculated as, width = width of the content, and height = height of the content (excluding the values of border and padding).
border-box
The width and height properties include the content, the padding and border, but not the margin. This is the box model used by Internet Explorer when the document is in Quirks mode. Note that padding and border will be inside of the box e.g.  .box {width: 350px; border: 10px solid black;} leads to a box rendered in the browser of width: 350px. The content box can't be negative and is floored to 0, making it impossible to use border-box to make the element disappear.
Here the dimension is calculated as, width = border + padding + width of the content, and height = border + padding + height of the content.
padding-box
The width and height properties include the content, the padding but neither the border, nor the margin. Only Firefox implemented this value, and it got removed in Friefox 50.

Formal syntax

content-box | border-box

Examples

/* support Firefox, Chrome, Safari, Opera, IE8+ and old Android */

.example {
  -moz-box-sizing: border-box;
  box-sizing: border-box;
}

Specifications

Specification Status Comment
CSS Basic User Interface Module Level 3
The definition of 'box-sizing' in that specification.
Candidate Recommendation Initial definition

Browser compatibility

Feature Chrome Edge Firefox (Gecko) Internet Explorer Opera Safari (WebKit)
Basic support 1.0 -webkit[1]
10.0
(Yes)

1.0 (1.7 or earlier)-moz[1]
29.0 (29.0)

8.0[1]

7.0 3.0 (522)-webkit
5.1[2]
padding-box No support No support 1.0 (1.7 or earlier)-moz[1]
29.0 (29.0)
Removed in 50.0 (50.0)
No support No support No support
Feature Android Firefox Mobile (Gecko) IE Phone Opera Mobile Safari Mobile
Basic support 2.1-webkit[1]
4.0
1.0 (1.0)-moz [1]
29.0 (29.0)
9.0 (Yes) (Yes)
padding-box No support 1.0 (1.0)-moz [1]
29.0 (29.0)
Removed in 50.0 (50.0)
No support No support No support

[1] box-sizing is not respected when the height is calculated from window.getComputedStyle(), in Internet Explorer (all versions), in Firefox prior to 23, and in Chrome. Edge doesn't exhibit the problem. Note that IE9's proprietary currentStyle property does return the correct value of height.

[2] The vendor prefix -webkit was removed in 534.12.

See also