box-sizing

The CSS box-sizing property is used to alter the default CSS box model used to calculate width and height of the elements.

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

In CSS, 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 is the default, and 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 value 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.

<div class="container">
<div class="parent">
  <p>Parent container</p>
  <div class="child" style="box-sizing: content-box;">
    <p>Child container</p>
    <p class="css">
    box-sizing: content-box;<br/>
    width: 100%;<br/>
    padding: 0;<br/>
    border-width: 0;</p>
  </div>
</div>
<div class="parent">
  <p>Parent container</p>
  <div class="child border-padding" style="box-sizing: content-box;">
    <p>Child container</p>
    <p class="css">
    box-sizing: content-box;<br/>
    width: 100%;<br/>
    padding: 5px;<br/>
    border-width: 10px;</p>
    </div>
</div>
<div class="parent">
  <p>Parent container</p>
  <div class="child border-padding"  style="box-sizing: border-box;">
    <p>Child container</p>
    <p class="css">
    box-sizing: border-box;<br/>
    width: 100%;<br/>
    padding: 5px;<br/>
    border-width: 10px;</p>
    </div>
</div>
</div>

p {
  margin-left: 0.2em;
  font-family: "Open Sans", sans-serif;
}
.container {
  display: flex;
}
.parent {
  width: 220px;
  border: solid 10px #FFC129;
  margin: 0.8em;
}
.child {
  width: 100%;
  margin: 2em auto;
  background-color: rgba(229, 232, 252, 0.6);
}
.border-padding {
  padding: 5px;
  border: solid #5B6DCD 10px;
}
.css {
  font-family: monospace;
}

The example above shows three scenarios. In each scenario there is a parent DIV (with an orange border) that contains a child DIV. The child has width: 100% set, and a pale blue background.

• The first scenario uses the default box-sizing: content-box. The child DIV has no padding and no border, and fits neatly inside its parent.
• The second scenario uses the default box-sizing: content-box. The child DIV has added padding and a border. The child then spills outside the parent because its width is calculated using only the content: padding and border are then added to make the rendered width.
• The third scenario uses box-sizing: border-box. The child DIV now fits neatly inside its parent because its width: 100% accounts for the padding and border.

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 (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. 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

HTML

<div class="content-box">Content box</div>
<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;
}
.border-box {
  box-sizing: border-box;
}

Result

Specifications

Browser compatibility

[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] In addition to the unprefixed support, Gecko 44.0 (Firefox 44.0 / Thunderbird 44.0 / SeaMonkey 2.41) added support for a -webkit prefixed version of the property for web compatibility reasons behind the preference layout.css.prefixes.webkit, defaulting to false. Since Gecko 49.0 (Firefox 49.0 / Thunderbird 49.0 / SeaMonkey 2.46) the preference defaults to true.

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

See also

• CSS box model