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.

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.


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

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

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

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


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

Formal definition

Initial valueauto
Applies tosame as width and height
Percentagesblock-size of containing block
Computed valuesame as width and height
Animation typea length, percentage or calc();

Formal syntax



Block size with vertical text


<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

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See also