The anchor-size() CSS function enables setting anchor-positioned element's size, position, and margins relative to the dimensions of anchor elements. It returns the <length> of a specified side of the target anchor element. anchor-size() is only valid when used within the value of anchor-positioned elements' sizing, inset, and margin properties.

For detailed information on anchor features and usage, see the CSS anchor positioning module landing page and the Using CSS anchor positioning guide.

Syntax

/* sizing relative to anchor side */
width: anchor-size(width);
block-size: anchor-size(block);
height: calc(anchor-size(self-inline) + 2em);

/* sizing relative to named anchor's side */
width: anchor-size(--my-anchor width);
block-size: anchor-size(--my-anchor block);

/* sizing relative to named anchor's side with fallback */
width: anchor-size(--my-anchor width, 50%);
block-size: anchor-size(--my-anchor block, 200px);

/* positioning relative to anchor side */
left: anchor-size(width);
inset-inline-end: anchor-size(--my-anchor height, 100px);

/* setting margin relative to anchor side */
margin-left: calc(anchor-size(width) / 4);
margin-block-start: anchor-size(--my-anchor self-block, 20px);

Parameters

The anchor-size() function's syntax is as follows:

anchor-size(<anchor-name> <anchor-size>, <length-percentage>)

The parameters are:

<anchor-name> Optional

The anchor-name property value of an anchor element you want to set the element's size, position, or margins relative to. This is a <dashed-ident> value. If omitted, the element's default anchor is used.

Note: Specifying an <anchor-name> inside an anchor-size() function neither associates nor tethers an element to an anchor; it only defines which anchor the element's property values should be set relative to.

<anchor-size> Optional

Specifies the dimension of the anchor element that the positioned element's property values will be set relative to. Valid values include:

width

The width of the anchor element.

height

The height of the anchor element.

block

The length of the anchor element's containing block in the block direction.

inline

The length of the anchor element's containing block in the inline direction.

self-block

The length of the anchor element in the block direction.

self-inline

The length of the anchor element in the inline direction.

Note: If this parameter is omitted, the dimension defaults to the <anchor-size> keyterm that matches the axis of the property in which the function is included. For example, width: anchor-size(); is equivalent to width: anchor-size(width);.

<length-percentage> Optional

Specifies the size to use as a fallback value if the element is not absolutely or fixed positioned, or the anchor element doesn't exist. If this parameter is omitted in a case when the fallback would otherwise be used, the declaration is invalid.

Return value

Returns a <length> value.

Description

The anchor-size() function enables a positioned element's sizing, position, and margin values to be expressed in terms of an anchor element's dimensions; it returns a <length> value representing the dimension of a specific anchor element the positioned element's property values are set relative to. It is a valid value for sizing, inset, and margin properties set on anchor-positioned elements.

The length returned is the vertical or horizontal size of an anchor element or its containing block. The dimension used is defined by the <anchor-size> parameter. If that parameter is omitted, the dimension used will match the axis of the sizing, position, or margin property is it set on. So for example:

• width: anchor-size() is equivalent to width: anchor-size(width).
• top: anchor-size() is equivalent to top: anchor-size(height).
• margin-inline-end: anchor-size() is equivalent to margin-inline-end: anchor-size(self-inline). margin-inline-end: anchor-size() is also equivalent to margin-inline-end: anchor-size(width) in horizontal writing modes, or margin-inline-end: anchor-size(height) in vertical writing modes.

The anchor element used as the basis for the dimension length is the element with the anchor-name specified in the <anchor-name> parameter. If more than one element has the same anchor name, the last element with that anchor name in the DOM order is used.

If no <anchor-name> parameter is included in the function call, the element's default anchor, referenced in its position-anchor property, or associated with the element via the anchor HTML attribute, is used.

If an <anchor-name> parameter is included and there are no elements matching that anchor name, the fallback value is used. If no fallback was included, the declaration is ignored. For example, if width: anchor-size(--foo width, 50px); height: anchor-size(--foo width); were specified on the positioned element but no anchor named --foo exists in the DOM, the width would be 50px and the height declaration would have no effect.

If an element has sizing, position, or margin properties with anchor-size() values set on them, but it is not an anchor-positioned element (it does not have its position property set to absolute or fixed or does not have an anchor associated with it via its position-anchor property), the fallback value will be used if one is available. If no fallback is available, the declaration is ignored.

For example, if width: anchor-size(width, 50px); were specified on the positioned element but no anchor was associated with it, the fallback value would be used, so width would get a computed value of 50px.

For detailed information on anchor features and usage, see the CSS anchor positioning module landing page and the Using CSS anchor positioning guide.

Properties that accept anchor-size() function values

The properties that accept an anchor-size() function as a value include:

• Sizing properties:
  • width
  • height
  • min-width
  • min-height
  • max-width
  • max-height
  • block-size
  • inline-size
  • min-block-size
  • min-inline-size
  • max-block-size
  • max-inline-size
• Inset properties:
  • bottom
  • left
  • right
  • top
  • inset shorthand
  • inset-block shorthand
  • inset-block-end
  • inset-block-start
  • inset-inline shorthand
  • inset-inline-end
  • inset-inline-start
• Margin properties:
  • margin shorthand
  • margin-bottom
  • margin-left
  • margin-right
  • margin-top
  • margin-block shorthand
  • margin-block-end
  • margin-block-start
  • margin-inline shorthand
  • margin-inline-end
  • margin-inline-start

Using anchor-size() inside calc()

The most common anchor-size() functions you'll use will just refer to a dimension of the default anchor. Alternative, include the anchor-size() function inside a calc() function to modify the size applied to the positioned element.

For example, this rule sizes the positioned element's width equal to the default anchor element's width:

.positionedElem {
  width: anchor-size(width);
}

This rule sizes the positioned element's inline size to 4 times the anchor element's inline size, with the multiplication being done inside a calc() function:

.positionedElem {
  inline-size: calc(anchor-size(self-inline) * 4);
}

Formal syntax

<anchor-size()> = 
  anchor-size( [ <anchor-name> || <anchor-size> ]? , <length-percentage>? )  

<anchor-name> = 
  <dashed-ident>  

<anchor-size> = 
  width        |
  height       |
  block        |
  inline       |
  self-block   |
  self-inline  

<length-percentage> = 
  <length>      |
  <percentage>  

Examples

Basic anchor-size() usage

This example shows two elements positioned relative to an anchor and sized using anchor-size() functions.

HTML

We specify three <div> elements, one anchor element and the two infobox elements we'll position relative to the anchor. We also include filler text to make the <body> tall enough to require scrolling, but this has been hidden for the sake of brevity.

<div class="anchor">⚓︎</div>

<div class="infobox" id="infobox1">
  <p>This is the first infobox.</p>
</div>

<div class="infobox" id="infobox2">
  <p>This is the second infobox.</p>
</div>

CSS

We declare the anchor <div> as an anchor element by giving it an anchor-name. The positioned elements, with their position properties set to fixed, are associated with the anchor element via their position-anchor properties. We also set absolute height and width dimensions on the anchor to provide a reference point when checking the positioned element dimensions, for example, with browser developer tools:

.anchor {
  anchor-name: --my-anchor;
  width: 100px;
  height: 100px;
}

.infobox {
  position-anchor: --my-anchor;
  position: fixed;
}

We set some distinct property values on the positioned elements:

• The positioned elements are tethered to the anchor with different position-area values that position the elements in different places around the anchor element.
• The height of the first infobox is set to the same height as the anchor element: anchor-size(height) returns the anchor element's height. The element's width is set to double the anchor element's width using the anchor-size() function within a calc() function: anchor-size(width) retrieves the anchor element's width, which is then multiplied by two.
• The height of the second infobox is set to two-thirds of the anchor element's height, using a similar technique.
• Margin values are included to provide some separation from the anchor element.

#infobox1 {
  position-area: right;
  height: anchor-size(height);
  width: calc(anchor-size(width) * 2);
  margin-left: 5px;
}

#infobox2 {
  position-area: top span-right;
  height: calc(anchor-size(height) / 1.5);
  margin-bottom: 5px;
}

Result

Use your browser tools to inspect the anchor-positioned elements. The first infobox will be 100px tall and 200px wide, while the second infobox will have a height of approximately 66.7px, with the width defaulting to max-content.

Position and margin example

See anchor-size() position and margin example.

Specifications

Browser compatibility

See also

• anchor-name
• position-anchor
• anchor() function
• Using CSS anchor positioning guide
• CSS anchor positioning module