The box-sizing
CSS property sets how the total width and height of an element is calculated.
By default in the CSS box model, 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. For example, if you have four boxes with width: 25%;
, if any has left or right padding or a left or right border, they will not by default fit on one line within the constraints of the parent container.
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, 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.
Note: It is often useful to set box-sizing
to border-box
to layout elements. This makes dealing with the sizes of elements much easier, and generally eliminates a number of pitfalls you can stumble on while laying out your content. On the other hand, when using position: relative
or position: absolute
, use of box-sizing: content-box
allows the positioning values to be relative to the content, and independent of changes to border and padding sizes, which is sometimes desirable.
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
andheight
properties include the content, but does not include the padding, border, or margin. For example,.box {width: 350px; border: 10px solid black;}
renders a box that is 370px wide. - 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
andheight
properties include the content, padding, and border, but do not include the margin. Note that padding and border will be inside of the box. For example,.box {width: 350px; border: 10px solid black;}
renders a box that is 350px wide, with the area for content being 330px wide. The content box can't be negative and is floored to 0, making it impossible to useborder-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 definition
Initial value | content-box |
---|---|
Applies to | all elements that accept width or height |
Inherited | no |
Computed value | as specified |
Animation type | discrete |
Formal syntax
content-box | border-box
Examples
Box sizes with content-box and border-box
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
Specification | Status | Comment |
---|---|---|
CSS Basic User Interface Module Level 3 The definition of 'box-sizing' in that specification. |
Recommendation | Initial definition. |
Browser compatibility
Desktop | Mobile | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
box-sizing | Chrome
Full support
10
| Edge
Full support
12
| Firefox
Full support
29
| IE
Full support
8
| Opera Full support 7 | Safari
Full support
5.1
| WebView Android
Full support
4
| Chrome Android
Full support
18
| Firefox Android
Full support
29
| Opera Android
Full support
14
| Safari iOS
Full support
6
| Samsung Internet Android
Full support
1.0
|
padding-box | Chrome No support No | Edge No support No | Firefox No support 1 — 50 | IE No support No | Opera No support No | Safari No support No | WebView Android No support No | Chrome Android No support No | Firefox Android No support 4 — 50 | Opera Android No support No | Safari iOS No support No | Samsung Internet Android No support No |
Legend
- Full support
- Full support
- No support
- No support
- Non-standard. Expect poor cross-browser support.
- Non-standard. Expect poor cross-browser support.
- Deprecated. Not for use in new websites.
- Deprecated. Not for use in new websites.
- See implementation notes.
- See implementation notes.
- User must explicitly enable this feature.
- User must explicitly enable this feature.
- Requires a vendor prefix or different name for use.
- Requires a vendor prefix or different name for use.