css · 2026-02-09

box-sizing: border-box in CSS

In CSS the default behavior is that padding and borders make elements take more space. For example:

div {
  width: 200px;
  border: 10px solid black;
}

The rendered div will actually be of 220px of height and width.

The behavior most people expect of padding and borders being part of the height and width defined is achieve by using box-sizing: border-box. With box-sizing: border-box:

• Width and height include content + padding + border
• Adding a border does not increase the outer size
• Content shrinks instead

Set predictable layouts

The snippet below applies box-sizing: border-box to all layout boxes making width and height predictable.

*,
*::before,
*::after {
  box-sizing: border-box;
}