How do I vertically align text in a div?

You need to add the line-height attribute and that attribute must match the height of the div. In your case:

.center {
  height: 309px;
  line-height: 309px; /* same as height! */
}

<div class="center">
  A single line.
</div>

In fact, you could probably remove the height attribute altogether.

This only works for one line of text though, so be careful.

The correct way to do this in modern browsers is to use Flexbox.

See this answer for details.

See below for some older ways that work in older browsers.

Vertical Centering in CSS
http://www.jakpsatweb.cz/css/css-vertical-center-solution.html

Article summary:

For a CSS 2 browser, one can use display:table/display:table-cell to center content.

A sample is also available at JSFiddle:

div { border:1px solid green;}

<div style="display: table; height: 400px; overflow: hidden;">
  <div style="display: table-cell; vertical-align: middle;">
    <div>
      everything is vertically centered in modern IE8+ and others.
    </div>
  </div>
</div>

It is possible to merge hacks for old browsers (Internet Explorer 6/7) into styles with using # to hide styles from newer browsers:

div { border:1px solid green;}

<div style="display: table; height: 400px; #position: relative; overflow: hidden;">
  <div style=
    "#position: absolute; #top: 50%;display: table-cell; vertical-align: middle;">
    <div style=" #position: relative; #top: -50%">
      everything is vertically centered
    </div>
  </div>
</div>