Within Container

Place your content block inside of a position: relative container to perfectly center your content within the container!

.Absolute-Center {
  width: 50%;
  height: 50%;
  overflow: auto;
  margin: auto;
  position: absolute;
  top: 0; left: 0; bottom: 0; right: 0;
}

Within Viewport

Set your content block to position: fixed and give it a z-index to keep it centered in the viewport.

• Mobile Safari: The content block will be centered vertically in the whole document, not the viewport, if it is not within a position: relative container.

.Absolute-Center.is-Fixed {
  width: 50%;
  height: 50%;
  overflow: auto;
  margin: auto;
  position: fixed;
  top: 0; left: 0; bottom: 0; right: 0;
  z-index: 999;
}

See the Modal Demo page.

Responsive

Perhaps the best benefit of Absolute Centering is that percentage-based width/heights work perfectly! Even min-width/max-width and min-height/max-height styles behave as expected for responsive boxes.

Go ahead, add padding to the element; Absolute Centering doesn’t mind!

.Absolute-Center.is-Responsive {
  width: 60%;
  height: 60%;
  min-width: 400px;
  max-width: 500px;
  padding: 40px;
  overflow: auto;
  margin: auto;
  position: absolute;
  top: 0; left: 0; bottom: 0; right: 0;
}

Offsets

If you have a fixed header or need to add other offsets, simply add it in your content block’s styles like top: 70px;. As long as margin: auto; is declared, the content block will be vertically centered within the bounds you declare with top left bottom and right.

You can also stick your content block to the right or left while keeping it vertically centered, using right: 0; left: auto; to stick to the right or left: 0; right: auto; to stick to the left.

.Absolute-Center.is-Right {
  width: 50%;
  height: 50%;
  margin: auto;
  overflow: auto;
  position: absolute;
  top: 0; left: auto; bottom: 0; right: 20px;
  text-align: right;
}

Overflow

Content taller than the block or container (viewport or a position: relative container) will overflow and may spill outside the content block and container or even be cut off. Simply adding overflow: auto will allow the content to scroll within the block as long as the content block itself isn’t taller than its container (perhaps by adding max-height: 100%; if you don’t have any padding on the content block itself).

.Absolute-Center.is-Overflow {
  width: 50%;
  height: 300px;
  max-height: 100%;
  margin: auto;
  overflow: auto;
  position: absolute;
  top: 0; left: 0; bottom: 0; right: 0;
}

Images

Images work too! Apply the class/style to the image itself and set height: auto; like you would with a responsively-sized image to let it resize with the container.

Note that height: auto; works for images, but causes a regular content block to stretch to fill the container unless you use the variable height technique. It’s likely that browsers have to calculate the height for the rendered image, so margin: auto; ends up working as if you’d declared the height, in all tested browsers.

.Absolute-Center.is-Image {
  width: 50%;
  height: auto;
  margin: auto;
  position: absolute;
  top: 0; left: 0; bottom: 0; right: 0;
}

Variable Height

Absolute Centering does require a declared height, however the height can be percentage-based and controlled by max-height. This makes it ideal for responsive scenarios, just make sure you set an appropriate overflow.

One way around the declared height is adding display: table, centering the content block regardless of content length.

Caveats:

This will break cross-browser compatibility. You may want to consider the Table-Cell method in this case.

• Firefox/IE8: Using display: table aligns the content block to the top, but is still centered horizontally.
• IE9/10: Using display: table aligns the content block to the top left.
• Mobile Safari: The content block is centered vertically, but becomes slightly off-center horizontally when using percentage based widths.

.Absolute-Center.is-Variable {
  display: table;
  width: 50%;
  overflow: auto;
  margin: auto;
  position: absolute;
  top: 0; left: 0; bottom: 0; right: 0;
}

Other Techniques

Absolute Centering works really well for many cases, but there are other methods that may fit more specific needs. The most commonly recommended methods are Negative Margins, Transforms, Table-Cell, and now Flexbox. These techniques are covered more in depth in other articles, so I’ll only go over the basics here.

.is-Negative {
        width: 300px;
        height: 200px;
        padding: 20px;
        position: absolute;
        top: 50%; left: 50%;
        margin-left: -170px; /* (width + padding)/2 */
        margin-top: -120px; /* (height + padding)/2 */
}
      

.is-Transformed {
	width: 50%;
	margin: auto;
	position: absolute;
	top: 50%; left: 50%;
	-webkit-transform: translate(-50%,-50%);
    -ms-transform: translate(-50%,-50%);
        transform: translate(-50%,-50%);
}
      

<div class="Pos-Container is-Table">
  <div class="Table-Cell">
    <div class="Center-Block">
    &lt!-- CONTENT -->
    </div>
  </div>
</div>
      

.Pos-Container.is-Table { display: table; }
.is-Table .Table-Cell {
  display: table-cell;
  vertical-align: middle;
}
.is-Table .Center-Block {
  width: 50%;
  margin: 0 auto;
}
      

.Pos-Container.is-Flexbox {
  display: -webkit-box;
  display: -moz-box;
  display: -ms-flexbox;
  display: -webkit-flex;
  display: flex;
  -webkit-box-align: center;
     -moz-box-align: center;
     -ms-flex-align: center;
  -webkit-align-items: center;
          align-items: center;
  -webkit-box-pack: center;
     -moz-box-pack: center;
     -ms-flex-pack: center;
  -webkit-justify-content: center;
          justify-content: center;
}
      

Summary

Each technique has its advantages and disadvantages. Which one you choose mainly boils down to which browsers you support and what your existing structure looks like.

Absolute Centering works great as a simple drop-in solution with no-fuss. Anywhere you used Negative Margins before, use Absolute Centering instead. You won’t have to deal with pesky math for the margins or extra markup, and you’ll be able to size your boxes responsively.

If you need variable height content cross-browser, try out the Table-Cell or Flexbox techniques to see what works best for your site.

(vf)