Fertile Window Calculator

About this calculator

Ovulation is estimated by counting backward from the next expected period: the luteal phase (the time between ovulation and menstruation) is consistently about 14 days for most cycle lengths, so ovulation day ≈ cycle length − 14, counted from the first day of the last period. Because sperm can survive in the reproductive tract for up to 5 days and the egg is viable for roughly 24 hours after release, the fertile window spans the 5 days leading up to ovulation plus the day itself.

This calculator takes your last period's start date and your average cycle length to mark that window on a calendar. People trying to conceive use it to time intercourse for the highest chance of pregnancy, while fertility awareness method (FAM) users use the same window, alongside other signs like basal body temperature, to avoid pregnancy naturally.