The border-bottom-width CSS property sets the width of the bottom border of a box.

/* Keyword values */
border-bottom-width: thin;
border-bottom-width: medium;
border-bottom-width: thick;

/* <length> values */
border-bottom-width: 10em;
border-bottom-width: 3vmax;
border-bottom-width: 6px;

/* Global keywords */
border-bottom-width: inherit;
border-bottom-width: initial;
border-bottom-width: unset;

Initial valuemedium
Applies toall elements. It also applies to ::first-letter.
Inheritedno
Mediavisual
Computed valuethe absolute length or 0 if border-bottom-style is none or hidden
Animation typea length
Canonical orderthe unique non-ambiguous order defined by the formal grammar

Syntax

Values

<br-width>
Defines the width of the border, either as an explicit nonnegative <length> or a keyword. If it's a keyword, it must be one of the following values:
thin
 
A thin border
medium
 
A medium border
thick
 
A thick border

Note: Because the specification doesn't define the exact thickness denoted by each keyword, the precise result when using one of them is implementation-specific. Nevertheless, they always follow the pattern thin ≤ medium ≤ thick, and the values are constant within a single document.

Formal syntax

<br-width>

where
<br-width> = <length> | thin | medium | thick

Example

HTML

<div>Element 1</div>
<div>Element 2</div>

CSS

div { 
  border: 1px solid red;
  margin: 1em 0;
}

div:nth-child(1) {
  border-bottom-width: thick;
}
div:nth-child(2) {
  border-bottom-width: 2em;
}

Result

Specifications

Specification Status Comment
CSS Backgrounds and Borders Module Level 3
The definition of 'border-bottom-width' in that specification.
Candidate Recommendation No significant change.
CSS Level 2 (Revision 1)
The definition of 'border-bottom-width' in that specification.
Recommendation No significant change.
CSS Level 1
The definition of 'border-bottom-width' in that specification.
Recommendation Initial definition.

Browser compatibility

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic Support1 (Yes)143.51
FeatureAndroidChrome for AndroidEdge mobileFirefox for AndroidIE mobileOpera AndroidiOS Safari
Basic Support2.3 ? (Yes)16111

See also