Ovulation Calculator and Fertile Window

This calculator uses the calendar method. Ovulation is taken as the luteal phase (usually 14 days) before the next period starts, and the fertile window as the six days ending on ovulation day, because sperm can survive up to five days and the egg about one (Wilcox, Weinberg and Baird, NEJM 1995). A longer or shorter cycle is mostly a longer or shorter phase before ovulation, so in a 32-day cycle ovulation falls about 18 days after the period starts.

For example, with a last period starting on 1 October and a 28-day cycle, ovulation is expected on 15 October, the fertile window runs from 10 to 15 October and the next period is due on 29 October. The table starts at the cycle that contains today and lists six.

The calendar method is an average-based estimate: ovulation can move by several days from one month to the next even in the same person, and the error grows when cycles are shorter than 21 days, longer than 35 or irregular. It is not accurate enough for contraception. When trying to conceive, ovulation test strips, basal body temperature or an ultrasound give a much better reading. The dates you enter stay in this browser and are never sent anywhere.