I think this would be the most accurate calculator.
It's usually better to remain loose on the due date though. It's just a guess. When you tell people the time you're due, I recommend sticking with a due month or due time. Like, "End of April, early May" for example.
Happily married Christian SAHM of 2 boys, DD1 , and DD2 July 2013
Jackie and Rafael (4/15/04)
DS (4/25/05) my unschooled airbending pokemon wizard
DD (10/05/06) my spirited pixie, who weaned at 3 yrs 10 months
DD (7/27/09) my UC water baby - I mean toddler!
DS (2/21/12) UC #2! My littlest love