The Box-Sizing CSS Property

CSS box sizing explained

The box-sizing CSS property is used to specify what exactly do the height and width values include:

div {
    box-sizing: border-box;
    width: 200px;
    height: 300px;
    border: 2px solid green;
}

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box-sizing: CSS syntax

There are two values you can use for the box-sizing CSS property:

box-sizing: content-box; / box-sizing: border-box;

.content-box { 
    box-sizing: content-box;
} 

.border-box { 
    box-sizing: border-box; 
}

content-box is the default value for box-sizing in CSS. It means the width and height values only include content. It does not include border, margin, or padding. If your element is set to be X pixels wide and Y pixels high, its final size with border and padding will actually take up more space.

border-box means the width and height values include content, border and padding. However, it doesn't include the margin. This box sizing option simplifies working with layouts, as you can always be sure of the final size.

See an example in the table below to get a better idea of box-sizing in CSS:

Note: the padding-box value was used for the box-sizing CSS property the past, but it's been deprecated since then.

Browser support

Chrome

10+

Edge

12+

Firefox

29+

IE

8+

Opera

7+

Safari

5.1+

Mobile browser support

Chrome

18+

Firefox

29+

Opera

All

Safari

All