How to get div height to auto-adjust to background size?

How do I get a div to automatically adjust to the size of the background I set for it without setting a specific height (or min-height) for it?

There is a very nice and cool way to make a background image work like an img element so it adjust its height automatically. You need to know the image width and height ratio. Set the height of the container to 0 and set the padding-top as percentage based upon the image ratio.

It will look like the following:

div {
    background-image: url('http://www.pets4homes.co.uk/images/articles/1111/large/feline-influenza-all-about-cat-flu-5239fffd61ddf.jpg');
    background-size: contain;
    background-repeat: no-repeat;
    width: 100%;
    height: 0;
    padding-top: 66.64%; /* (img-height / img-width * container-width) */
                /* (853 / 1280 * 100) */
}

You just got a background image with auto height which will work just like an img element. Here is a working prototype (you can resize and check the div height): http://jsfiddle.net/8m9ur5qj/

Another, perhaps inefficient, solution would be to include the image under an img element set to visibility: hidden;. Then make the background-image of the surrounding div the same as the image.

This will set the surrounding div to the size of the image in the img element but display it as a background.

<div style="background-image: url(http://your-image.jpg);">
 <img src="http://your-image.jpg" style="visibility: hidden;" />
</div>

There is no way to auto adjust for background image size using CSS.

You can hack around it by measuring the background image on the server and then applying those attributes to the div, as others have mentioned.

You could also hack up some javascript to resize the div based on the image size (once the image has been downloaded) - this is basically the same thing.

If you need your div to auto-fit the image, I might ask why don't you just put an <img> tag inside your div?

This answer is similar to others, but is overall the best for most applications. You need to know the image size before hand which you usually do. This will let you add overlay text, titles etc. with no negative padding or absolute positioning of the image. They key is to set the padding % to match the image aspect ratio as seen in the example below. I used this answer and essentially just added an image background.

.wrapper {
  width: 100%;
  /* whatever width you want */
  display: inline-block;
  position: relative;
  background-size: contain;
  background: url('https://upload.wikimedia.org/wikipedia/en/thumb/6/67/Wiki-llama.jpg/1600px-Wiki-llama.jpg') top center no-repeat;
  margin: 0 auto;
}
.wrapper:after {
  padding-top: 75%;
  /* this llama image is 800x600 so set the padding top % to match 600/800 = .75 */
  display: block;
  content: '';
}
.main {
  position: absolute;
  top: 0;
  bottom: 0;
  right: 0;
  left: 0;
  color: black;
  text-align: center;
  margin-top: 5%;
}

<div class="wrapper">
  <div class="main">
    This is where your overlay content goes, titles, text, buttons, etc.
  </div>
</div>