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



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

Formal syntax




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


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


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

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic support No ?


38 — 511

No No No
FeatureAndroid webviewChrome for AndroidEdge mobileFirefox for AndroidOpera AndroidiOS SafariSamsung Internet
Basic support No ? ?


38 — 511

No No No

1. 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.

See also

Document Tags and Contributors

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