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The preserveAspectRatio attribute indicates how an element with a viewBox providing a given aspect ratio must fit into a viewport with a different aspect ratio.

Because the aspect ratio of an SVG image is define by the viewBox attribute, if this attribute isn't set, the preserveAspectRatio atttribute has no effect (with one exception, the <image> element, as described below).

<svg viewBox="-1 -1 162 92" xmlns="http://www.w3.org/2000/svg">
  <defs>
     <path id="smiley" d="M50,10 A40,40,1,1,1,50,90 A40,40,1,1,1,50,10 M30,40 Q36,35,42,40 M58,40 Q64,35,70,40 M30,60 Q50,75,70,60 Q50,75,30,60" />
  </defs>
 
  <rect x="0" y="0" width="20" height="10" />
  <svg viewBox="0 0 100 100" width="20" height="10"
       preserveAspectRatio="xMidYMid meet" x="0" y="0">
    <use href="#smiley" />
  </svg>
 
  <rect x="25" y="0" width="20" height="10" />
  <svg viewBox="0 0 100 100" width="20" height="10"
       preserveAspectRatio="xMinYMid meet" x="25" y="0">
    <use href="#smiley" />
  </svg>
 
  <rect x="50" y="0" width="20" height="10" />
  <svg viewBox="0 0 100 100" width="20" height="10"
       preserveAspectRatio="xMaxYMid meet" x="50" y="0">
    <use href="#smiley" />
  </svg>
 
  <rect x="0" y="15" width="20" height="10" />
  <svg viewBox="0 0 100 100" width="20" height="10"
       preserveAspectRatio="xMidYMin slice" x="0" y="15">
    <use href="#smiley" />
  </svg>
 
  <rect x="25" y="15" width="20" height="10" />
  <svg viewBox="0 0 100 100" width="20" height="10"
       preserveAspectRatio="xMidYMid slice" x="25" y="15">
    <use href="#smiley" />
  </svg>
 
  <rect x="50" y="15" width="20" height="10" />
  <svg viewBox="0 0 100 100" width="20" height="10"
       preserveAspectRatio="xMidYMax slice" x="50" y="15">
    <use href="#smiley" />
  </svg>
 
  <rect x="75" y="0" width="10" height="25" />
  <svg viewBox="0 0 100 100" width="10" height="25"
       preserveAspectRatio="xMidYMin meet" x="75" y="0">
    <use href="#smiley" />
  </svg>
 
  <rect x="90" y="0" width="10" height="25" />
  <svg viewBox="0 0 100 100" width="10" height="25"
       preserveAspectRatio="xMidYMid meet" x="90" y="0">
    <use href="#smiley" />
  </svg>
 
  <rect x="105" y="0" width="10" height="25" />
  <svg viewBox="0 0 100 100" width="10" height="25"
       preserveAspectRatio="xMidYMax meet" x="105" y="0">
    <use href="#smiley" />
  </svg>

  <rect x="120" y="0" width="10" height="25" />
  <svg viewBox="0 0 100 100" width="10" height="25"
       preserveAspectRatio="xMinYMid slice" x="120" y="0">
    <use href="#smiley" />
  </svg>
 
  <rect x="135" y="0" width="10" height="25" />
  <svg viewBox="0 0 100 100" width="10" height="25"
       preserveAspectRatio="xMidYMid slice" x="135" y="0">
    <use href="#smiley" />
  </svg>
 
  <rect x="150" y="0" width="10" height="25" />
  <svg viewBox="0 0 100 100" width="10" height="25"
       preserveAspectRatio="xMaxYMid slice" x="150" y="0">
    <use href="#smiley" />
  </svg>
 
  <rect x="0" y="30" width="160" height="60" />
  <svg viewBox="0 0 100 100" width="160" height="60"
       preserveAspectRatio="none" x="0" y="30">
    <use href="#smiley" />
  </svg>
</svg>
path {
  fill: yellow;
  stroke: black;
  stroke-width: 8px;
  stroke-linecap: round;
  stroke-linejoin: round;
  pointer-events: none;
}

rect:hover, rect:active {
  outline: 1px solid red;
}

Seven elements are using this attribute: <svg>, <symbol>, <image>, <feImage>, <marker>, <pattern>, and <view>.

Its value is made of one or two keywords: A required alignment value and an optional "meet or slice" reference as described below:

Alignment value
The alignment value indicates whether to force uniform scaling and, if so, the alignment method to use in case the aspect ratio of the viewBox doesn't match the aspect ratio of the viewport. The alignment value must be one of the following keywords:
  • none
    Do not force uniform scaling. Scale the graphic content of the given element non-uniformly if necessary such that the element's bounding box exactly matches the viewport rectangle. Note that if <align> is none, then the optional <meetOrSlice> value is ignored.
  • xMinYMin - Force uniform scaling.
    Align the <min-x> of the element's viewBox with the smallest X value of the viewport.
    Align the <min-y> of the element's viewBox with the smallest Y value of the viewport.
  • xMidYMin - Force uniform scaling.
    Align the midpoint X value of the element's viewBox with the midpoint X value of the viewport.
    Align the <min-y> of the element's viewBox with the smallest Y value of the viewport.
  • xMaxYMin - Force uniform scaling.
    Align the <min-x>+<width> of the element's viewBox with the maximum X value of the viewport.
    Align the <min-y> of the element's viewBox with the smallest Y value of the viewport.
  • xMinYMid - Force uniform scaling.
    Align the <min-x> of the element's viewBox with the smallest X value of the viewport.
    Align the midpoint Y value of the element's viewBox with the midpoint Y value of the viewport.
  • xMidYMid (the default) - Force uniform scaling.
    Align the midpoint X value of the element's viewBox with the midpoint X value of the viewport.
    Align the midpoint Y value of the element's viewBox with the midpoint Y value of the viewport.
  • xMaxYMid - Force uniform scaling.
    Align the <min-x>+<width> of the element's viewBox with the maximum X value of the viewport.
    Align the midpoint Y value of the element's viewBox with the midpoint Y value of the viewport.
  • xMinYMax - Force uniform scaling.
    Align the <min-x> of the element's viewBox with the smallest X value of the viewport.
    Align the <min-y>+<height> of the element's viewBox with the maximum Y value of the viewport.
  • xMidYMax - Force uniform scaling.
    Align the midpoint X value of the element's viewBox with the midpoint X value of the viewport.
    Align the <min-y>+<height> of the element's viewBox with the maximum Y value of the viewport.
  • xMaxYMax - Force uniform scaling.
    Align the <min-x>+<width> of the element's viewBox with the maximum X value of the viewport.
    Align the <min-y>+<height> of the element's viewBox with the maximum Y value of the viewport.
Meet or slice reference
The meet or slice reference is optional and, if provided, must be one of the following keywords:
  • meet (the default) - Scale the graphic such that:
    • aspect ratio is preserved
    • the entire viewBox is visible within the viewport
    • the viewBox is scaled up as much as possible, while still meeting the other criteria
    In this case, if the aspect ratio of the graphic does not match the viewport, some of the viewport will extend beyond the bounds of the viewBox (i.e., the area into which the viewBox will draw will be smaller than the viewport).
  • slice - Scale the graphic such that:
    • aspect ratio is preserved
    • the entire viewport is covered by the viewBox
    • the viewBox is scaled down as much as possible, while still meeting the other criteria
    In this case, if the aspect ratio of the viewBox does not match the viewport, some of the viewBox will extend beyond the bounds of the viewport (i.e., the area into which the viewBox will draw is larger than the viewport).

feImage

For <feImage>, preserveAspectRatio defines how the referenced image should be fit in the rectangle define by the <feImage> element.

Value <align> <meetOrSlice>?
Default value xMidYMid meet
Animatable Yes

image

For <feImage>, preserveAspectRatio defines how the referenced image should be fit in the rectangle define by the <image> element.

Value <align> <meetOrSlice>?
Default value xMidYMid meet
Animatable Yes

marker

For <marker>, preserveAspectRatio indicates if a uniform scaling must be performed to fit the element viewport.

Value <align> <meetOrSlice>?
Default value xMidYMid meet
Animatable Yes

pattern

For <pattern>, preserveAspectRatio indicates if a uniform scaling must be performed to fit the element viewport.

Value <align> <meetOrSlice>?
Default value xMidYMid meet
Animatable Yes

svg

For <svg>, preserveAspectRatio indicates if a uniform scaling must be performed to fit the element viewport.

Value <align> <meetOrSlice>?
Default value xMidYMid meet
Animatable Yes

symbol

For <symbol>, preserveAspectRatio indicates if a uniform scaling must be performed to fit the element viewport.

Value <align> <meetOrSlice>?
Default value xMidYMid meet
Animatable Yes

view

For <view>, preserveAspectRatio indicates if a uniform scaling must be performed to fit the element viewport.

Value <align> <meetOrSlice>?
Default value xMidYMid meet
Animatable Yes

Specification

Specification Status Comment
Filter Effects Module Level 1
The definition of 'preserveAspectRatio' in that specification.
Working Draft  
Scalable Vector Graphics (SVG) 2
The definition of 'preserveAspectRatio' in that specification.
Candidate Recommendation  
Scalable Vector Graphics (SVG) 1.1 (Second Edition)
The definition of 'preserveAspectRatio' in that specification.
Recommendation Initial definition

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