Average according to surveys tends to be about 7 inches. Average according to actual measurements is about 5. The latter is obviously a more reliable source. Most people who measure themselves (or the guys they are with) tend to exaggerate, estimate (with a predisposition for higher numbers), round up or measure incorrectly.
In short, if you're not lying (and probably even if you are lying a little) then you're at or above average and have nothing to worry about.