`block-size` 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 September 2021.

The block-size CSS property defines the size of an element's block along the block axis. If the writing-mode is horizontal, it corresponds to the height; if the writing mode is vertical, it corresponds to the width. A related property is inline-size, which defines the other dimension of the element.

Try it

block-size: 150px;
writing-mode: horizontal-tb;

block-size: 150px;
writing-mode: vertical-rl;

block-size: auto;
writing-mode: horizontal-tb;

block-size: auto;
writing-mode: vertical-lr;

<section class="default-example" id="default-example">
  <div class="transition-all" id="example-element">
    This is a box where you can change the block-size.
  </div>
</section>

#example-element {
  display: flex;
  flex-direction: column;
  background-color: #5b6dcd;
  justify-content: center;
  color: white;
}

Syntax

/* <length> values */
block-size: 300px;
block-size: 25em;
block-size: anchor-size(height);
block-size: calc(anchor-size(--my-anchor block) * 0.75);

/* <percentage> values */
block-size: 75%;

/* Keyword values */
block-size: max-content;
block-size: min-content;
block-size: fit-content;
block-size: fit-content(20em);
block-size: auto;

/* Global values */
block-size: inherit;
block-size: initial;
block-size: revert;
block-size: revert-layer;
block-size: unset;

Values

The block-size property takes the same values as the width and height properties.

Formal definition

Initial value	auto
Applies to	same as `width` and `height`
Inherited	no
Percentages	block-size of containing block
Computed value	same as `width` and `height`
Animation type	$1$, <a href="/en-US/docs/Web/CSS/Reference/Values/percentage#interpolation" title="Values of the <percentage> CSS data type are interpolated as real, floating-point numbers.">percentage</a> or calc();

Formal syntax

block-size = 
  <'width'>

Examples

Block size with vertical text

HTML

<p class="exampleText">Example text</p>

CSS

.exampleText {
  writing-mode: vertical-rl;
  background-color: yellow;
  block-size: 200px;
}

Result

Specifications

Specification
CSS Logical Properties and Values Module Level 1 # propdef-block-size

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
`block-size`	57 8	79 79	41	44 15	12.1 5.1	57 18	41	43 14	12.2 5	7.0 1.0	57 4.4	12.2 5
`anchor-size`	125	125	147	111	26	125	147	83	26	27.0	125	26
`auto`	8	79	41	15	5.1	18	41	14	5	1.0	4.4	5
`fit-content`	57	79	94 41	44	12.1	57	94 41	43	12.2	7.0	57	12.2
`fit-content_function`	No	No	91	No	No	No	No	No	No	No	No	No
`max-content`	57	79	66 41	44	12.1	57	66 41	43	12.2	5.0	57	12.2
`min-content`	57	79	66 41	44	12.1	57	66 41	43	12.2	5.0	57	12.2
`stretch`	138	138	No	122	27	138	No	91	27	30.0	138	27