You basically need to set top and left to 50% to center the left-top corner of the div. You also need to set the margin-top and margin-left to the negative half of the div’s height and width to shift the center towards the middle of the div.
Thus, provided a <!DOCTYPE html> (standards mode), this should do:
position: fixed; width: 500px; height: 200px; top: 50%; left: 50%; margin-top: -100px; /* Negative half of height. */ margin-left: -250px; /* Negative half of width. */
Or, if you don’t care about centering vertically and old browsers such as IE6/7, then you can instead also add left: 0 and right: 0 to the element having a margin-left and margin-right of auto, so that the fixed positioned element having a fixed width knows where its left and right offsets start. In your case thus:
position: fixed; width: 500px; height: 200px; margin: 5% auto; /* Will not center vertically and won't work in IE6/7. */ left: 0; right: 0;
Again, this works only in IE8+ if you care about IE, and this centers only horizontally not vertically.