The border-bottom-width CSS property sets the width of the bottom border of an element.


/* 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;


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
  • medium
  • thick

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 definition

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

Formal syntax


<line-width> = <length> | thin | medium | thick


Comparing bottom border widths


<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

BCD tables only load in the browser

See also