From Smashing Magazine

.Absolute-Center {
  margin: auto;
  position: absolute;
  top: 0; left: 0; bottom: 0; right: 0;
}

Advantages:

• Cross-browser (including IE8-10 without hacks!)
• No special markup, minimal styles
• Responsive with percentages and min-/max-
• Centered regardless of padding (without box-sizing!)
• Works great on images

Caveats:

• Height must be declared (see Variable Height)
• Recommend setting overflow: auto to prevent content spillover (see Overflow)

See this Fiddle