This is an experimental technology
Check the Browser compatibility table carefully before using this in production.

The block-size CSS property defines the horizontal or vertical size of an element's block, depending on its writing mode. It corresponds to either the width or the height property, depending on the value of writing-mode.

/* <length> values */
block-size: 300px;
block-size: 25em;

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

/* Keyword values */
block-size: 25em border-box;
block-size: 75% content-box;
block-size: max-content;
block-size: min-content;
block-size: available;
block-size: fit-content;
block-size: auto;

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

If the writing mode is vertically oriented, the value of block-size relates to the width of the element; otherwise, it relates to the height of the element. A related property is inline-size, which defines the other dimension of the element.

Initial valueauto
Applies tosame as width and height
Inheritedno
Percentagesblock-size of containing block
Mediavisual
Computed valuesame as width and height
Animation typediscrete
Canonical orderthe unique non-ambiguous order defined by the formal grammar

Syntax

Values

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

Formal syntax

<'width'>

Example

HTML

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

CSS

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

Specification

Specification Status Comment
CSS Logical Properties and Values Level 1
The definition of 'block-size' in that specification.
Editor's Draft Initial definition

Browser compatibility

Update compatibility data on GitHub
DesktopMobile
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidEdge MobileFirefox for AndroidOpera for AndroidiOS SafariSamsung Internet
Basic support
Experimental
Chrome No support NoEdge ? Firefox Full support 41
Full support 41
No support 38 — 51
Disabled
Disabled From version 38 until version 51 (exclusive): this feature is behind the layout.css.vertical-text.enabled preference (needs to be set to true). To change preferences in Firefox, visit about:config.
IE No support NoOpera No support NoSafari No support NoWebView Android No support NoChrome Android ? Edge Mobile ? Firefox Android Full support 41
Full support 41
No support 38 — 51
Disabled
Disabled From version 38 until version 51 (exclusive): this feature is behind the layout.css.vertical-text.enabled preference (needs to be set to true). To change preferences in Firefox, visit about:config.
Opera Android No support NoSafari iOS No support NoSamsung Internet Android No support No

Legend

Full support  
Full support
No support  
No support
Compatibility unknown  
Compatibility unknown
Experimental. Expect behavior to change in the future.
Experimental. Expect behavior to change in the future.
User must explicitly enable this feature.
User must explicitly enable this feature.

See also

Document Tags and Contributors

Contributors to this page: fscholz, mfluehr, elysdir1, teoli, HTMLValidator, Sebastianz, jsx, hbloomer
Last updated by: fscholz,