Maintain aspect ratio of div but fill screen width and height in CSS?

Solution 1:

Use the new CSS viewport units vw and vh (viewport width / viewport height)

FIDDLE

Resize vertically and horizontally and you'll see that the element will always fill the maximum viewport size without breaking the ratio and without scrollbars!

(PURE) CSS

div
{
    width: 100vw; 
    height: 56.25vw; /* height:width ratio = 9/16 = .5625  */
    background: pink;
    max-height: 100vh;
    max-width: 177.78vh; /* 16/9 = 1.778 */
    margin: auto;
    position: absolute;
    top:0;bottom:0; /* vertical center */
    left:0;right:0; /* horizontal center */
}

* {
  margin: 0;
  padding: 0;
}
div {
  width: 100vw;
  height: 56.25vw;
  /* 100/56.25 = 1.778 */
  background: pink;
  max-height: 100vh;
  max-width: 177.78vh;
  /* 16/9 = 1.778 */
  margin: auto;
  position: absolute;
  top: 0;
  bottom: 0;
  /* vertical center */
  left: 0;
  right: 0;
  /* horizontal center */
}

<div></div>

If you want to use a maximum of say 90% width and height of the viewport: FIDDLE

* {
  margin: 0;
  padding: 0;
}
div {
  width: 90vw;
  /* 90% of viewport vidth */
  height: 50.625vw;
  /* ratio = 9/16 * 90 = 50.625 */
  background: pink;
  max-height: 90vh;
  max-width: 160vh;
  /* 16/9 * 90 = 160 */
  margin: auto;
  position: absolute;
  top: 0;
  bottom: 0;
  left: 0;
  right: 0;
}

<div></div>

Also, browser support is pretty good too: IE9+, FF, Chrome, Safari- caniuse

Solution 2:

Just reformulating Danield's answer in a LESS mixin, for further usage:

// Mixin for ratio dimensions    
.viewportRatio(@x, @y) {
  width: 100vw;
  height: @y * 100vw / @x;
  max-width: @x / @y * 100vh;
  max-height: 100vh;
}

div {

  // Force a ratio of 5:1 for all <div>
  .viewportRatio(5, 1);

  background-color: blue;
  margin: 0;
  position: absolute;
  top: 0; right: 0; bottom: 0; left: 0;
}

Solution 3:

Use a CSS media query @media together with the CSS viewport units vw and vh to adapt to the aspect ratio of the viewport. This has the benefit of updating other properties, like font-size, too.

This fiddle demonstrates the fixed aspect ratio for the div, and the text matching its scale exactly.

Initial size is based on full width:

div {
  width: 100vw; 
  height: calc(100vw * 9 / 16);

  font-size: 10vw;

  /* align center */
  margin: auto;
  position: absolute;
  top: 0px; right: 0px; bottom: 0px; left: 0px;

  /* visual indicators */
  background:
    url(data:image/gif;base64,R0lGODlhAgAUAIABAB1ziv///yH5BAEAAAEALAAAAAACABQAAAIHhI+pa+EPCwA7) repeat-y center top,
    url(data:image/gif;base64,R0lGODlhFAACAIABAIodZP///yH5BAEAAAEALAAAAAAUAAIAAAIIhI8Zu+nIVgEAOw==) repeat-x left center,
    silver;
}

Then, when the viewport is wider than the desired aspect ratio, switch to height as base measure:

/* 100 * 16/9 = 177.778 */
@media (min-width: 177.778vh) {
  div {
    height: 100vh;
    width: calc(100vh * 16 / 9);

    font-size: calc(10vh * 16 / 9);
  }
}

This is very similar in vein to the max-width and max-height approach by @Danield, just more flexible.