background-size

Summary

The background-size CSS property specifies the size of the background images. The size of the image can be fully constrained or only partially in order to preserve its intrinsic ratio.

Syntax

/* Keywords syntax */
background-size: cover;
background-size: contain;

/* One-value syntax */
/* the width of the image (height set to 'auto') */
background-size: 50%;
background-size: 3em;
background-size: 12px;
background-size: auto;

/* Two-value syntax */
/* first value: width of the image, second value: height */
background-size: 50% auto;
background-size: 3em 25%;
background-size: auto 6px;
background-size: auto auto;

/* Multiple backgrounds values by background-image */
/* Do not confuse this with background-size: auto auto */
background-size: auto, auto;
background-size: 50%, 25%, 25%;
background-size: 6px, auto, contain;

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

Values

<length>

A <length> value that scales the background image to the specified length in the corresponding dimension. Negative lengths are not allowed.

<percentage>

A <percentage> value that scales the background image in the corresponding dimension to the specified percentage of the background positioning area, which is determined by the value of background-origin. The background positioning area is, by default, the area containing the content of the box and its padding; the area may also be changed to just the content or to the area containing borders, padding, and content. If the background's attachment is fixed, the background positioning area is instead the entire area of the browser window, not including the area covered by scrollbars if they are present. Negative percentages are not allowed.

auto

A keyword that scales the background image in the corresponding direction such that its intrinsic proportion is maintained.

contain

A keyword that scales the image as large as possible and maintains image aspect ratio (image doesn't get squished). Image is letterboxed within the container. When the image and container have different dimensions, the empty areas (either top/bottom of left/right) are filled with the background-color.  The image is automatically centered unless over-ridden by another property such as background-position.

cover

A keyword that is the inverse of contain. Scales the image as large as possible and maintains image aspect ratio (image doesn't get squished). The image "covers" the entire width or height of the container. When the image and container have different dimensions, the image is clipped either left/right or top/bottom.

The interpretation of possible values depends on the image's intrinsic dimensions (width and height) and intrinsic proportion (ratio of width and height).  A bitmap image always has intrinsic dimensions and an intrinsic proportion.  A vector image may have both intrinsic dimensions (and thus must have an intrinsic proportion).  It also may have one or no intrinsic dimensions, and in either case it might or might not have an intrinsic proportion.  Gradients are treated as images with no intrinsic dimensions or intrinsic proportion.

Background images generated from elements using -moz-element (which actually match an element) are currently treated as images with the dimensions of the element, or of the background positioning area if the element is SVG, with the corresponding intrinsic proportion.

The rendered size of the background image is then computed as follows:

If both components of background-size are specified and are not auto:

The background image renders at the specified size.

If the background-size is contain or cover:

The image is rendered, preserving its intrinsic proportion, at the largest size contained within, or covering, the background positioning area.  If the image has no intrinsic proportion, then it is rendered at the size of the background positioning area.

If the background-size is auto or auto auto:

If the image has both intrinsic dimensions, it is rendered at that size.  If it has no intrinsic dimensions and no intrinsic proportion, it is rendered at the size of the background positioning area.  If it has no dimensions but has a proportion, it's rendered as if contain had been specified instead.  If the image has one intrinsic dimension and a proportion, it's rendered at the size determined by that one dimension and the proportion.  If the image has one intrinsic dimension but no proportion, it's rendered using the intrinsic dimension and the corresponding dimension of the background positioning area.

If the background-size has one auto component and one non-auto component:

If the image has an intrinsic proportion, then render it using the specified dimension, and compute the other dimension from the specified dimension and the intrinsic proportion.  If the image has no intrinsic proportion, use the specified dimension for that dimension.  For the other dimension, use the image's corresponding intrinsic dimension if there is one.  If there is no such intrinsic dimension, use the corresponding dimension of the background positioning area.

Note that background sizing for vector images that lack intrinsic dimensions or proportion is not yet fully implemented in all browsers.  Be careful about relying on the behavior described above, and test in multiple browsers (specifically including versions of Firefox 7 or earlier and Firefox 8 or greater) to be sure different renderings are acceptable.

Formal syntax

<bg-size>#
where 
<bg-size> = [ <length> | <percentage> | auto ]{1,2} | cover | contain

Examples

This demonstration of background-size: cover and this demonstration of background-size: contain are meant to be opened in new windows so that you can see how contain and cover behave when the background positioning area's dimensions vary. This series of demos of how background-size works and interacts with other background-* properties should pretty much cover the remaining ground in how to use background-size alone and in conjunction with other properties.

Notes

If you are specifying a gradient as background and have specified a background-size to go with it, it's best not to specify a size that uses a single auto component, or is specified using only a width value (for example, background-size: 50%).  Rendering of gradients in such cases changed in Firefox 8, and at present it is generally inconsistent across browsers, which do not all implement rendering in full accordance with the CSS3 background-size specification and with the CSS3 Image Values gradient specification.

.bar {
       width: 50px; height: 100px;
       background-image: gradient(...);

       /* NOT RECOMMENDED */
       background-size: 25px;
       background-size: 50%;
       background-size: auto 50px;
       background-size: auto 50%;

       /* OKAY */
       background-size: 25px 50px;
       background-size: 50% 50%;
}

Note that it's particularly not recommended to use a pixel dimension and an auto dimension with a gradient, because it's impossible to replicate rendering in versions of Firefox prior to 8, and in browsers not implementing Firefox 8's rendering, without knowing the exact size of the element whose background is being specified.

Specifications

Browser compatibility

• [1] Opera 9.5's computation of the background positioning area is incorrect for fixed backgrounds.  Opera 9.5 also interprets the two-value form as a horizontal scaling factor and, from appearances, a vertical clipping dimension. This has been fixed in Opera 10.
• [2] WebKit-based browsers originally implemented an older draft of CSS3 background-size in which an omitted second value is treated as duplicating the first value; this draft does not include the contain or cover keywords.
• [3] Konqueror 3.5.4 supports -khtml-background-size.
• [4] While this property is new in Gecko 1.9.2 (Firefox 3.6), it is possible to stretch a image fully over the background in Firefox 3.5 by using -moz-border-image.

  .foo {
    background-image: url(bg-image.png);
  
    -webkit-background-size: 100% 100%;           /* Safari 3.0 */
       -moz-background-size: 100% 100%;           /* Gecko 1.9.2 (Firefox 3.6) */
         -o-background-size: 100% 100%;           /* Opera 9.5 */
            background-size: 100% 100%;           /* Gecko 2.0 (Firefox 4.0) and other CSS3-compliant browsers */
   
    -moz-border-image: url(bg-image.png) 0;    /* Gecko 1.9.1 (Firefox 3.5) */
  }

• [5] Though Internet Explorer 8 doesn't support the background-size property, it is possible to emulate some of its functionality using the non-standard -ms-filter function:

  -ms-filter: "progid:DXImageTransform.Microsoft.AlphaImageLoader(src='path_relative_to_the_HTML_file', sizingMethod='scale')";

  This simulates the value cover.

See also

• CSS Reference
• Multiple backgrounds
• Scaling background images