background-size - CSS: Cascading Style Sheets 编辑

The background-size CSS property sets the size of the element's background image. The image can be left to its natural size, stretched, or constrained to fit the available space.

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Spaces not covered by a background image are filled with the background-color property, and the background color will be visible behind background images that have transparency/translucency.

Syntax

/* Keyword values */
background-size: cover;
background-size: contain;

/* One-value syntax */
/* the width of the image (height becomes 'auto') */
background-size: 50%;
background-size: 3.2em;
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 */
background-size: auto, auto; /* Not to be confused with `auto auto` */
background-size: 50%, 25%, 25%;
background-size: 6px, auto, contain;

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

The background-size property is specified in one of the following ways:

• Using the keyword values contain or cover.
• Using a width value only, in which case the height defaults to auto.
• Using both a width and a height value, in which case the first sets the width and the second sets the height. Each value can be a <length>, a <percentage>, or auto.

To specify the size of multiple background images, separate the value for each one with a comma.

Values

contain

Scales the image as large as possible within its container without cropping or stretching the image. If the container is larger than the image, this will result in image tiling, unless the background-repeat property is set to no-repeat.

cover

Scales the image as large as possible to fill the container, stretching the image if necessary. If the proportions of the image differ from the element, it is cropped either vertically or horizontally so that no empty space remains.

auto

Scales the background image in the corresponding direction such that its intrinsic proportions are maintained.

<length>

Stretches the image in the corresponding dimension to the specified length. Negative values are not allowed.

<percentage>

Stretches the image in the corresponding dimension to the specified percentage of the background positioning area. The background positioning area is determined by the value of background-origin (by default, the padding box). However, if the background's background-attachment value is fixed, the positioning area is instead the entire viewport. Negative values are not allowed.

Intrinsic dimensions and proportions

The computation of values depends on the image's intrinsic dimensions (width and height) and intrinsic proportions (width-to-height ratio). These attributes are as follows:

• A bitmap image (such as JPG) always has intrinsic dimensions and proportions.
• A vector image (such as SVG) does not necessarily have intrinsic dimensions. If it has both horizontal and vertical intrinsic dimensions, it also has intrinsic proportions. If it has no dimensions or only one dimension, it may or may not have proportions.
• CSS <gradient>s have no intrinsic dimensions or intrinsic proportions.
• Background images created with the element()() function use the intrinsic dimensions and proportions of the generating element.

Note: The behavior of <gradient>s changed in Gecko 8.0 (Firefox 8.0 / Thunderbird 8.0 / SeaMonkey 2.5). Before this, they were treated as images with no intrinsic dimensions, but with intrinsic proportions identical to that of the background positioning area.

Note: In Gecko, background images created using the element()() function 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. This is non-standard behavior.

Based on the intrinsic dimensions and proportions, the rendered size of the background image is computed as follows:

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

The background image is rendered at the specified size.

If the background-size is contain or cover:

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

If the background-size is auto or auto auto:

• If the image has both horizontal and vertical intrinsic dimensions, it's rendered at that size.
• If the image has no intrinsic dimensions and has no intrinsic proportions, it's rendered at the size of the background positioning area.
• If the image has no intrinsic dimensions but has intrinsic proportions, it's rendered as if contain had been specified instead.
• If the image has only one intrinsic dimension and has intrinsic proportions, it's rendered at the size corresponding to that one dimension. The other dimension is computed using the specified dimension and the intrinsic proportions.
• If the image has only one intrinsic dimension but has no intrinsic proportions, it's rendered using the specified dimension and the other dimension of the background positioning area.

Note: SVG images have a preserveAspectRatio attribute that defaults to the equivalent of contain. In Firefox 43, as opposed to Chrome 52, an explicit background-size causes preserveAspectRatio to be ignored.

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

• If the image has intrinsic proportions, it's stretched to the specified dimension. The unspecified dimension is computed using the specified dimension and the intrinsic proportions.
• If the image has no intrinsic proportions, it's stretched to the specified dimension. The unspecified dimension is computed using the image's corresponding intrinsic dimension, if there is one. If there is no such intrinsic dimension, it becomes the corresponding dimension of the background positioning area.

Note: Background sizing for vector images that lack intrinsic dimensions or proportions is not yet fully implemented in all browsers. Be careful about relying on the behavior described above, and test in multiple browsers to be sure the results are acceptable.

Working with gradients

If you use a <gradient> as the background and specify 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 <gradient>s in such cases changed in Firefox 8, and at present 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.

.gradient-example {
  width: 50px;
  height: 100px;
  background-image: linear-gradient(blue, red);

  /* Not safe to use */
  background-size: 25px;
  background-size: 50%;
  background-size: auto 50px;
  background-size: auto 50%;

  /* Safe to use */
  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.

Formal definition

Formal syntax

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

Examples

Tiling a large image

Let's consider a large image, a 2982x2808 Firefox logo image. We want to tile four copies of this image into a 300x300-pixel element. To do this, we can use a fixed background-size value of 150 pixels.

HTML

<div class="tiledBackground">
</div>

CSS

.tiledBackground {
  background-image: url(https://www.wenjiangs.com/wp-content/uploads/2020/mozilla/logo-quantum.9c5e96634f92.png);
  background-size: 150px;
  width: 300px;
  height: 300px;
  border: 2px solid;
  color: pink;
}

Result

See Scaling background images for more examples.

Specifications

Browser compatibility

BCD tables only load in the browser

The compatibility table in this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request.

See also

• Scaling background images
• Scaling of SVG backgrounds
• object-fit