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.

/* 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: revert;
background-size: revert-layer;
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.

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 (while preserving its ratio) to the smallest possible size to fill the container (that is: both its height and width completely cover the container), leaving no empty space. If the proportions of the background differ from the element, the image is cropped either vertically or horizontally.

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.

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.

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; 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.

background-size = 
  <bg-size>#  

<bg-size> = 
  [ <length-percentage [0,∞]> | auto ]{1,2}  |
  cover                                      |
  contain                                    

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

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.mozilla.org/media/img/logos/firefox/logo-quantum.9c5e96634f92.png);
  background-size: 150px;
  width: 300px;
  height: 300px;
  border: 2px solid;
  color: pink;
}

Result

See Resizing background images for more examples.

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