Actual Stat = (((2 * Base Stat + IV + (EV / 4)) * Level / 100 + 5) * Nature)
where Nature = 1.1 if the Pokemon's nature boosts the stat (like Timid), or 0.9 if it hinders the stat (like Quiet), or 1 if it doesn't do anything to the stat.
See
http://www.smogon.com/dp/articles/stats for more info.