CSS background-size

The CSS background-size property was introduced in CSS3 for the purposes of setting the dimensions of a background image.

The background-size property allows you to specify exactly what size your background image is. Furthermore, you can use the cover and contain keywords to scale the image to the box.

The background-size property is used in conjunction with the background-image property to define a background image on an HTML element.

In CSS3, the background-size property has been added to the background shorthand property. This means that you can set the background-size from within the background property (for example, background: url("background.png") 40% / 10em lightblue round fixed border-box;).

Syntax

background-size: value

The formal syntax for this property is:

background-size: <bg-size> [ , <bg-size> ]*

Where

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

See below for more information.

Possible Values

Here are the possible values:

contain
Specifies that the background image should be scaled, while preserving its intrinsic aspect ratio (if any), to the largest size such that both its width and its height can fit inside the background positioning area.
cover
Specifies that the background image should be scaled, while preserving its intrinsic aspect ratio (if any), to the smallest size such that both its width and its height can completely cover the background positioning area.
<length>

Specifies the size explicitly using a lenth value. For example, 300px 100px will result in a background image being scaled to 300 pixels wide and 100 pixels high.

The first value provided defines the width of the background image. The second value provides the height. If only one value is given, the second becomes auto.

For example, background-size:200px 50px provides a width of 200 pixels and a height of 50 pixels. A value of background-size:200px will result in the width being 200 pixels and the height being automatically determined by using the image's intrinsic ratio and the size of the other dimension, or failing that, using the image's intrinsic size, or failing that, treating it as 100%. In other words, the image should keep its proportions as it scales (i.e. not get distorted).

<percentage>

Specifies the size explicitly using a percentage value. A percentage value is relative to the background positioning area - not to the original image size. For example, 70% will result in a background image being scaled to 70 percent of the background positioning area.

As with the <length> values, the first value provides the width and the second provides the height. If the second value is missing, the height is automatically determined.

auto
Specifies that the image size is automatically determined using the intrinsic width and/or height of the image.

Basic Property Information

Initial Value
auto
Applies To
All elements
Inherited?
No
Media
Visual
Animatable
Yes, except keyword values (see example)

Example Code

Basic CSS

background-size: contain;

Working Example within an HTML Document

<!DOCTYPE html>
<title>Example</title>
<style>
.background {
  width: 220px;
  height: 150px;
  padding: 10px;
  background-color: yellowgreen;
  background-image: url(/pix/samples/bg1.gif);
  border: 20px solid rgba(0,0,0,0.3);
  background-size: contain;
}
</style>
<div class="background">
Testing the <code>background-size</code> property!
</div>
<p>Change the value to see the difference between the <code>contain</code> and <code>cover</code> keywords, as well as providing an explicit size.</p>

Try it with the Online Editor

CSS Specifications

Browser Support

The following table provided by Caniuse.com shows the level of browser support for this feature.

Vendor Prefixes

For maximum browser compatibility many web developers add browser-specific properties by using extensions such as -webkit- for Safari, Google Chrome, and Opera (newer versions), -ms- for Internet Explorer, -moz- for Firefox, -o- for older versions of Opera etc. As with any CSS property, if a browser doesn't support a proprietary extension, it will simply ignore it.

This practice is not recommended by the W3C, however in many cases, the only way you can test a property is to include the CSS extension that is compatible with your browser.

The major browser manufacturers generally strive to adhere to the W3C specifications, and when they support a non-prefixed property, they typically remove the prefixed version. Also, W3C advises vendors to remove their prefixes for properties that reach Candidate Recommendation status.

Many developers use Autoprefixer, which is a postprocessor for CSS. Autoprefixer automatically adds vendor prefixes to your CSS so that you don't need to. It also removes old, unnecessary prefixes from your CSS.

You can also use Autoprefixer with preprocessors such as Less and Sass.