Est. Pregnancy Week?!!

Jordan • Married 6 years | Two Girls 4 & 2 | Baby Case due July
Totally confused.. So when you estimate conception based on last period why do the calculators add 1 or 2 weeks?? If conception date is estimated at 2/1/15 why do sites says pregnancy week is 6 weeks?? Would that only be 4 weeks since conception?? Thanks ladies!!