Our ovulation calculator predicts your fertile window — the days when you are most likely to conceive — based on the first day of your last period and your average cycle length. Ovulation typically occurs around 14 days before the start of your next period. Sperm can survive in the reproductive tract for up to 5 days, so the fertile window is typically the 5 days before ovulation plus the day of ovulation itself. The calculator gives you your predicted ovulation date and the 6-day fertile window for the current cycle. For the most accurate results, track your cycle over several months to establish your average cycle length.