Box Sizing

Overview

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, making the element wider than 100px.

border-box tells the browser to account for any border and padding in the values you specify for an element’s 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.

Values

content-box

Initial and default value specified by the CSS Standard.

The width and height properties include the content, but does not include the padding, border or margin.

For example, the following renders a box that is 370px wide.

.box {
  width: 350px;
  border: 10px solid black;
}

The dimensions of the element are calculated as:

• width = width of the content
• height = height of content

Borders and paddings are not included in the calculation.

border-box

The width and height properties include the content, padding and border but does not include the margin.

For example the following renders a box that is 350px wide. The area for content being 330px wide.

.box {
  width: 350px;
  border: 10px solid black;
}

Here the dimensions of the element are calculated as:

• width = border + padding + width of the content
• height = border + padding + height of the content