`border-bottom-width` CSS property

Baseline Widely available

This feature is well established and works across many devices and browser versions. It's been available across browsers since July 2015.

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

Try it

border-bottom-width: thick;

border-bottom-width: 2em;

border-bottom-width: 4px;

border-bottom-width: 2ex;

border-bottom-width: 0;

<section class="default-example" id="default-example">
  <div class="transition-all" id="example-element">
    This is a box with a border around it.
  </div>
</section>

#example-element {
  background-color: palegreen;
  color: black;
  border: 0 solid crimson;
  padding: 0.75em;
  width: 80%;
  height: 100px;
}

Syntax

/* 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: revert;
border-bottom-width: revert-layer;
border-bottom-width: unset;

Values

<line-width>: Defines the width of the border, either as an explicit non-negative <length> or the keywords: thin, medium, or thick. The default is medium.

Formal definition

Initial value	medium
Applies to	all elements
Inherited	no
Computed value	the absolute `<length>`, snapped as a line width
Animation type	a <a href="/en-US/docs/Web/CSS/Reference/Values/length#interpolation" title="Values of the <length> CSS data type are interpolated as real, floating-point numbers.">length</a>

Formal syntax

border-bottom-width = 
  <line-width>

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

Examples

Comparing bottom border widths

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
CSS Backgrounds and Borders Module Level 3 # border-width

Browser compatibility

	Desktop					Mobile
	Chrome	Edge	Firefox	Opera	Safari	Chrome Android	Firefox for Android	Opera Android	Safari on iOS	Samsung Internet	WebView Android	WebView on iOS
`border-bottom-width`	1	12	1	3.5	1	18	4	10.1	1	1.0	2.2	1
`medium`	1	12	1	15	1	18	4	14	1	1.0	4.4	1
`thick`	1	12	1	15	1	18	4	14	1	1.0	4.4	1
`thin`	1	12	1	15	1	18	4	14	1	1.0	4.4	1