box-sizing

The box-sizing CSS property defines how the the user agent should calculate the total width and height of an element.

In the CSS box model, by default, the width and height you assign to an element is applied only to the element's content box. If the element has any border or padding, this is then added to the width and height to arrive at the size of the box that's rendered on the screen. This means that when you set width and height you have to adjust the value you give to allow for any border or padding that may be added. This is especially tricky when implementing a responsive design.

The box-sizing property can be used to adjust this behavior:

• content-box gives you the default CSS box-sizing behavior. If you set an element's width to 100 pixels, then the element's content box will be 100 pixels wide, and the width of any border or padding will be added to the final rendered width.
• border-box tells the browser to account for any border and padding in the values you specify for width and height. If you set an element's width to 100 pixels, that 100 pixels will include any border or padding you added, and the content box will shrink to absorb that extra width. This typically makes it much easier to size elements.

Some experts recommend that web developers should consider routinely applying box-sizing: border-box to all elements.

Syntax

The box-sizing property is specified as a single keyword chosen from the list of values below.

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. For example, if you set .box {width: 350px;}, then apply {border: 10px solid black;} , then the rendered result is a box of width: 370px.

Here the dimensions of the element are calculated as: width = width of the content, and height = height of the content. (Borders and padding are not included in the calculation.)

border-box

The width and height properties include the content, the padding and border, but not the margin. 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 dimensions of the element are calculated as: width = border + padding + width of the content, and height = border + padding + height of the content.

Formal syntax

content-box | border-box

Example

This example shows how different box-sizing values alter the rendered size of two otherwise identical elements.

HTML

<div class="content-box">Content box</div>
<br>
<div class="border-box">Border box</div>

CSS

div {
  width: 160px;
  height: 80px;
  padding: 20px;
  border: 8px solid red;
  background: yellow;
}

.content-box { 
  box-sizing: content-box; 
  /* Total width: 160px + (2 * 20px) + (2 * 8px) = 216px
     Total height: 80px + (2 * 20px) + (2 * 8px) = 136px
     Content box width: 160px
     Content box height: 80px */
}

.border-box { 
  box-sizing: border-box;
  /* Total width: 160px
     Total height: 80px
     Content box width: 160px - (2 * 20px) - (2 * 8px) = 104px
     Content box height: 80px - (2 * 20px) - (2 * 8px) = 24px */
}

Result

Specifications

Browser compatibility

See also

• CSS box model