Scale div to fit background image

Question

I have a div with a background image that I want to expand 100% width and auto scale the div to fit the required height of the image. At the moment it is not scaling the div

Answer 1

body{ margin: 0; padding: 0}

#box1{
background: url(http://lorempixel.com/400/200/food/);
background-size: cover; 
background-attachment: fixed; 
margin: 0; 
width: 100%; 
height: 100vh; 
display: table;
}
h1{ color: #ffffff; font-family: "Gill Sans", "Gill Sans MT", "Myriad Pro", "DejaVu Sans Condensed", Helvetica, Arial, "sans-serif"; font-size: 38px; text-align: center; font-weight: normal; background: rgba(0,0,0,0.3); display: table-cell; vertical-align: middle}

<div id="box1">
		<h1>Code Bluster BILU </h1>
	</div>

Answer 2

If you know the proportions of the image, use percentage padding to define the height of the container. Set height:0 and set vertical padding to a percentage of the width.

They key to this method is that percentage-based vertical padding is always related to width.

According to the box model (w3.org):

The percentage is calculated with respect to the width of the generated box's containing block, even for 'padding-top' and 'padding-bottom'.

Below, the image is 400px X 200px, so the proportion of height to width is 1:2 and padding-top is set to 50%;

#mainHeaderWrapper {
  width: 100%;
  height: 0;
  padding-top:50%;
  background-image: url(//lorempixel.com/400/200/abstract/1/);
  background-size: 100% auto;
  background-repeat: no-repeat;
}

<div id="mainHeaderWrapper"></div>
stuff below the image

In another example, the image is 300px X 100px. The height is ⅓ of the width, so the padding-top is set to 33.33%:

#mainHeaderWrapper {
  width: 100%;
  height: 0;
  padding-top:33.33%;
  background-image: url(//lorempixel.com/300/100/abstract/1/);
  background-size: 100% auto;
  background-repeat: no-repeat;
}

<div id="mainHeaderWrapper"></div>
stuff below the image

---

Edit:

As prompted by Paulie_D, other content in the div must be positioned absolutely, demonstrated below. I suggest positioning these elements using percentages, as well.

#mainHeaderWrapper {
  width: 100%;
  height: 0;
  padding-top: 33.33%;
  background-image: url(//lorempixel.com/300/100/abstract/1/);
  background-size: 100% auto;
  background-repeat: no-repeat;
}
div#inner_content {
  position: absolute;
  top:0;
  width:100%;
  margin-top:10%;
  color: #FFF;
  text-align: center;
  font-size:20px;
}

<div id="mainHeaderWrapper">
  <div id="inner_content">Hello World</div>
</div>
stuff below the image

Answer 3

Let a transparent image dictate your DIV dimensions.

jsBin demo

Inside that div put the same image

<div id="mainHeaderWrapper">
   <img src="path/to/image.jpg"><!-- I'm invisible! yey!-->
</div>

set that image to

#mainHeaderWrapper{
    background: no-repeat url(path/to/image.jpg) 50% / 100%;
}
#mainHeaderWrapper img{
    vertical-align: top;
    width: 100%; /* max width */
    opacity: 0;  /* make it transparent */
}

That way the height of the DIV will be dictated by the containing invisible image,
Having the background-image set to center, full (50% / 100%) it will match that image's proportions.

Not sure if you want to manipulate the colors of the background-image or what (using CSS3 filter), but now after reading again what I wrote...
Isn't it simpler to simply put an image inside a DIV? :)

Need some content inside that DIV?

jsBin demo Due to the containing image, you'll need an extra child element that will be set to position:absolute and act as an overlay element

<div id="mainHeaderWrapper">
   <img src="path/to/image.jpg"><!-- I'm invisible! yey!-->
   <div>
     Some content...
   </div>
</div>

#mainHeaderWrapper{
    position: relative;
    background: no-repeat url(path/to/image.jpg) 50% / 100%;
}
#mainHeaderWrapper > img{
    vertical-align: top;
    width: 100%; /* max width */
    opacity: 0;  /* make it transparent */
}
#mainHeaderWrapper > div{
    position: absolute;
    top: 0;
    width: 100%;
    height: 100%;
}

Answer 4

This can be done without using a dummy image. I will use dimensions of an image I just worked with for example.

The dimensions of my image are 2880x1410. Simplify the dimensions -> 96/47 (I used this simple ratio calculator http://andrew.hedges.name/experiments/aspect_ratio/). Once you have the simplified ratio, plug the height and width to the equation:

height: calc((100vw * W) / H); So mine would read: height: calc((100vw * 47) / 96);

No need to worry about the contents of the div either (unless they dont fit)