Responsively change div size keeping aspect ratio [duplicate]

You can do this using pure CSS; no JavaScript needed. This utilizes the (somewhat counterintuitive) fact that padding-top percentages are relative to the containing block's width. Here's an example:

.wrapper {
  width: 50%;
  /* whatever width you want */
  display: inline-block;
  position: relative;
}
.wrapper:after {
  padding-top: 56.25%;
  /* 16:9 ratio */
  display: block;
  content: '';
}
.main {
  position: absolute;
  top: 0;
  bottom: 0;
  right: 0;
  left: 0;
  /* fill parent */
  background-color: deepskyblue;
  /* let's see it! */
  color: white;
}

<div class="wrapper">
  <div class="main">
    This is your div with the specified aspect ratio.
  </div>
</div>

Bumming off Chris's idea, another option is to use pseudo elements so you don't need to use an absolutely positioned internal element.

<style>
.square {
    /* width within the parent.
       can be any percentage. */
    width: 100%;
}
.square:before {
    content: "";
    float: left;

    /* essentially the aspect ratio. 100% means the
       div will remain 100% as tall as it is wide, or
       square in other words.  */
    padding-bottom: 100%;
}
/* this is a clearfix. you can use whatever
   clearfix you usually use, add
   overflow:hidden to the parent element,
   or simply float the parent container. */
.square:after {
    content: "";
    display: table;
    clear: both;
}
</style>
<div class="square">
  <h1>Square</h1>
  <p>This div will maintain its aspect ratio.</p>
</div>

I've put together a demo here: http://codepen.io/tcmulder/pen/iqnDr

EDIT:

Now, bumming off of Isaac's idea, it's easier in modern browsers to simply use vw units to force aspect ratio (although I wouldn't also use vh as he does or the aspect ratio will change based on window height).

So, this simplifies things:

<style>
.square {
    /* width within the parent (could use vw instead of course) */
    width: 50%;
    /* set aspect ratio */
    height: 50vw;
}
</style>
<div class="square">
  <h1>Square</h1>
  <p>This div will maintain its aspect ratio.</p>
</div>

I've put together a modified demo here: https://codepen.io/tcmulder/pen/MdojRG?editors=1100

You could also set max-height, max-width, and/or min-height, min-width if you don't want it to grow ridiculously big or small, since it's based on the browser's width now and not the container and will grow/shrink indefinitely.

Note you can also scale the content inside the element if you set the font size to a vw measurement and all the innards to em measurements, and here's a demo for that: https://codepen.io/tcmulder/pen/VBJqLV?editors=1100

<style>
#aspectRatio
{
  position:fixed;
  left:0px;
  top:0px;
  width:60vw;
  height:40vw;
  border:1px solid;
  font-size:10vw;
}
</style>
<body>
<div id="aspectRatio">Aspect Ratio?</div>
</body>

The key thing to note here is vw = viewport width, and vh = viewport height

Responsively change div size keeping aspect ratio [duplicate]

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