CSS box-sizing

<!DOCTYPE html>
<title>Example</title>
<style>
div {
  background: gold; 
  width: 150px; 
  height: 150px; 
  padding: 10px;
  border: 20px solid black;
  margin: 10px;
  float: left;
}
.content-box {
  box-sizing: content-box;
}
.border-box {
  box-sizing: border-box;
}
</style>
<div class="content-box">
  content-box
</div>
<div class="border-box">
  border-box
</div>

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

Initial Value: content-box
Applies To: All elements that accept height or width
Inherited?: No
Media: Visual
Animatable: No

Example Code

Basic CSS

Working Example within an HTML Document

Try it

CSS Specifications

The box-sizing property is defined in CSS Basic User Interface Module Level 3 (CSS3 UI) (W3C Candidate Recommendation, 7 July 2015)

Browser Support

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

Vendor Prefixes

For maximum browser compatibility many web developers add browser-specific properties by using extensions such as -webkit- for Safari, Google Chrome, and Opera (newer versions), -ms- for Internet Explorer, -moz- for Firefox, -o- for older versions of Opera etc. As with any CSS property, if a browser doesn't support a proprietary extension, it will simply ignore it.

This practice is not recommended by the W3C, however in many cases, the only way you can test a property is to include the CSS extension that is compatible with your browser.

The major browser manufacturers generally strive to adhere to the W3C specifications, and when they support a non-prefixed property, they typically remove the prefixed version. Also, W3C advises vendors to remove their prefixes for properties that reach Candidate Recommendation status.

Many developers use Autoprefixer, which is a postprocessor for CSS. Autoprefixer automatically adds vendor prefixes to your CSS so that you don't need to. It also removes old, unnecessary prefixes from your CSS.

You can also use Autoprefixer with preprocessors such as Less and Sass.

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