Conception Calculator
Conception happens on ovulation day, and a due date is simply conception + 266 days (38 weeks). So the two convert into each other exactly: pick whichever date you know.
Or, if you have a positive test: Pregnant? Calculate your due date →
Why conception + 266 days?
Obstetric dating counts pregnancy from the first day of the last period (LMP), giving the familiar 280 days / 40 weeks. But conception itself happens at ovulation, about 14 days after the LMP in a textbook 28-day cycle. The actual time from conception to birth averages about 266 days, so: due date = LMP + 280 = conception + 266. Both directions on this page use that identity — no other assumptions are hidden inside.
How precise is a "conception date"?
If the starting point is a due date, the result inherits the due date's own uncertainty: an LMP-based due date assumes a 28-day cycle with day-14 ovulation, which is off by days for most people, and even an early-ultrasound due date carries a margin of roughly ±5–7 days. Read the conception date as the centre of a window a few days wide, not a pinpoint. Note also that conception day is ovulation day — intercourse that led to it may have happened up to five days earlier, which is the fertile window shown above.