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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.
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



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:
A thin border
A medium border
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> = <length> | thin | medium | thick



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


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

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



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

Feature Chrome Edge Firefox (Gecko) Internet Explorer Opera Safari (WebKit)
Basic support 1.0 (Yes) 1.0 (1.0) 4.0 3.5 1.0 (85)
Feature Android Edge Firefox Mobile (Gecko) IE Phone Opera Mobile Safari Mobile
Basic support 2.3 (Yes) 1.0 (1.0) 6.0 11 1.0

See also