I’m using Twitter Bootstrap 3, and I have problems when I want to align vertically two div, for example — JSFiddle link:

<!-- Latest compiled and minified CSS -->
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.4/css/bootstrap.min.css">

<!-- Optional theme -->
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.4/css/bootstrap-theme.min.css">

<!-- Latest compiled and minified JavaScript -->
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.4/js/bootstrap.min.js"></script>
<div class="row">
  <div class="col-xs-5">
    <div style="height:5em;border:1px solid #000">Big</div>
  </div>
  <div class="col-xs-5">
    <div style="height:3em;border:1px solid #F00">Small</div>
  </div>
</div>

The grid system in Bootstrap uses float: left, not display:inline-block, so the property vertical-align doesn’t work. I tried using margin-top to fix it, but I think this is not a good solution for the responsive design.

Flexible box layout

With the advent of the CSS Flexible Box, many of web designers’ nightmares¹ have been resolved. One of the most hacky ones, the vertical alignment. Now it is possible even in unknown heights.

“Two decades of layout hacks are coming to an end. Maybe not tomorrow,
but soon, and for the rest of our lives.”

— CSS Legendary Eric Meyer at W3Conf 2013

Flexible Box (or in short, Flexbox), is a new layout system that is specifically designed for layout purposes. The specification states:

Flex layout is superficially similar to block layout. It lacks many of
the more complex text- or document-centric properties that can be used
in block layout, such as floats and columns. In return it gains simple
and powerful tools for distributing space and aligning content in ways
that webapps and complex web pages often need.

How can it help in this case? Well, let’s see.

Vertical aligned columns

Using Twitter Bootstrap we have .rows having some .col-*s. All we need to do is to display the desired .row² as a flex container box and then align all its flex items (the columns) vertically by align-items property.

EXAMPLE HERE (Please read the comments with care)

<div class="container">
    <div class="row vertical-align"> <!--
                    ^--  Additional class -->
        <div class="col-xs-6"> ... </div>
        <div class="col-xs-6"> ... </div>
    </div>
</div>

.vertical-align {
    display: flex;
    align-items: center;
}

The Output

Colored area displays the padding-box of columns.

Clarifying on align-items: center

8.3 Cross-axis Alignment: the align-items property

Flex items can be aligned in the cross axis of the current line of the flex container, similar to justify-content but in the perpendicular direction. align-items sets the default alignment for all of the flex container’s items, including anonymous flex items.

align-items: center;
By center value, the flex item’s margin box is centered in the cross axis within the line.

Big Alert

Important note #1: Twitter Bootstrap doesn’t specify the width of columns in extra small devices unless you give one of .col-xs-# classes to the columns.

Therefore in this particular demo, I have used .col-xs-* classes in order for columns to be displayed properly in mobile mode, because it specifies the width of the column explicitly.

But alternatively you could switch off the Flexbox layout simply by changing display: flex; to display: block; in specific screen sizes. For instance:

/* Extra small devices (767px and down) */
@media (max-width: 767px) {
    .row.vertical-align {
        display: block; /* Turn off the flexible box layout */
    }
}

Or you could specify .vertical-align only on specific screen sizes like so:

/* Small devices (tablets, 768px and up) */
@media (min-width: 768px) {
    .row.vertical-align {
        display: flex;
        align-items: center;
    }
}

In that case, I’d go with @KevinNelson’s approach.

Important note #2: Vendor prefixes omitted due to brevity. Flexbox syntax has been changed during the time. The new written syntax won’t work on older versions of web browsers (but not that old as Internet Explorer 9! Flexbox is supported on Internet Explorer 10 and later).

This means you should also use vendor-prefixed properties like display: -webkit-box and so on in production mode.

If you click on “Toggle Compiled View” in the Demo, you’ll see the prefixed version of CSS declarations (thanks to Autoprefixer).

Full-height columns with vertical aligned contents

As you see in the previous demo, columns (the flex items) are no longer as high as their container (the flex container box. i.e. the .row element).

This is because of using center value for align-items property. The default value is stretch so that the items can fill the entire height of the parent element.

In order to fix that, you can add display: flex; to the columns as well:

EXAMPLE HERE (Again, mind the comments)

.vertical-align {
  display: flex;
  flex-direction: row;
}

.vertical-align > [class^="col-"],
.vertical-align > [class*=" col-"] {
  display: flex;
  align-items: center;     /* Align the flex-items vertically */
  justify-content: center; /* Optional, to align inner flex-items
                              horizontally within the column  */
}

The Output

Colored area displays the padding-box of columns.

Last, but not least, notice that the demos and code snippets here are meant to give you a different idea, to provide a modern approach to achieve the goal. Please mind the “Big Alert” section if you are going to use this approach in real world websites or applications.

For further reading including browser support, these resources would be useful:

• Mozilla Developer Network – Flexible boxes
• Guide To Flexbox – CSS Tricks
• HTML5Rocks – Flexbox quick
• SmashingMagazine – Centering Elements with Flexbox
• Philip Walton – Solved By Flexbox
• Can I Use: Flexible Box Layout Module

<sup>1. Vertically align an image inside a div with responsive height
2. It’s better to use an additional class in order not to alter Twitter Bootstrap’s default .row.</sup>