CSS box-sizing

The box-sizing property specifies how an element's dimensions are calculated. It changes the CSS box model used to calculate heights and widths of elements.

The default CSS box model uses content-box, which means that any dimensions that you provide are used to specify the size of the content box only — any padding and borders are drawn outside of the content box. Therefore applying a width of 200px and padding of 10px to an element would result in the element occupying a total width of 220 pixels (assuming padding is added to all four sides, or at least, both left and right sides). This is because the padding is added to the box width.

You can use a value of border-box to change this, so that any padding and borders are included in any height and width dimensions. In this case, the element will take up only 200 pixels, because the padding is added to the inside of the element.

Syntax

Possible Values

content-box

This value results in the element's height and width dimensions (i.e. width, height, min-width, min-height, max-width, and max-height) being applied to the element's content box. Any padding and border is drawn outside of the content area.

This results in a box that is larger than what is specified in the height and width dimensions (assuming padding and/or border values are greater than 0).

border-box

This value results in the element's height and width dimensions being applied to the element's border box. Any padding and border is drawn inside the content area. The padding and border values are subtracted from the height and width dimensions.

This results in a box that is exactly the size that is specified in the height and width dimensions (regardless of any padding and border values).

In addition, all CSS properties also accept the following CSS-wide keyword values as the sole component of their property value:

initial

Represents the value specified as the property's initial value.

inherit

Represents the computed value of the property on the element's parent.

unset

This value acts as either inherit or initial, depending on whether the property is inherited or not. In other words, it sets all properties to their parent value if they are inheritable or to their initial value if not inheritable.

Basic Property Information

Example Code

Basic CSS

Working Example within an HTML Document

CSS Specifications

• The box-sizing property is defined in CSS Box Sizing Module Level 3 (W3C Working Draft)

Browser Support

Browser support for CSS properties can vary, particularly for newer or experimental features. Check current browser compatibility before using a property in production, especially when supporting older browsers.

The following table provided by Caniuse.com shows the level of browser support for this feature.

Initial Value

CSS properties have an initial value that applies when the property has not been explicitly set. The initial value is different from the value inherited from a parent element and may vary between properties.

Inheritance

Some CSS properties inherit their computed value from their parent element by default, while others do not. The inherit, initial, unset, and revert keywords can also be used to control how a property gets its value.

Vendor Prefixes

Most modern CSS properties use the standard, unprefixed syntax. Vendor-prefixed properties such as -webkit- are mainly relevant to older browser versions or specific features and should only be used when required by your browser support targets.