It depends. I actually calculated your due date online at the babycenter website...it came up with Sept 14, but it is based on a 28 day cycle.
If your cycle is predictible, and regular, you add 2 weeks to the first day of your LMP (last menstrual period)and your conception date is on that day, or give or take a few days. If your cycle is shorter or longer, you need to adjust accordingly.