It’s easy to calculate your due date based on one piece of information. You just need to know the first day of your last menstrual period.
Let’s take an example date of May 9th, 2013 as the first day of your last menstrual period.
Subtract 3 months from the Month -> May – 3 = Feb
Add 7 days to the day the 9th in the example -> 9 + 7 = 16
Your expected due date in the above example would be Feb 16th, 2014
This formula is called Nagele’s formula
About 50% women deliver in the range of EDD calculated with this method +/- 7 days.