Ovulation Window Calculator

The ovulation window calculator pinpoints your fertile window — the days each cycle when pregnancy is possible — using your last menstrual period (LMP) date and average cycle length. Ovulation releases a single egg that remains viable for only 12–24 hours, but because sperm can survive in the reproductive tract for up to five days in fertile cervical mucus, the fertile window actually spans about six days: the five days before ovulation and ovulation day itself. Getting the timing right matters for both conception planning and cycle awareness. Ovulation is triggered by a surge in luteinizing hormone (LH) that occurs 24–36 hours before the egg is released. The luteal phase constant — approximately 14 days — means ovulation falls 14 days before your next expected period regardless of cycle length. For a 28-day cycle that is day 14; for a 35-day cycle it is day 21. This calculator returns the actual calendar dates of your fertile window, peak ovulation day, and next period start so you can plan accordingly. For the most reliable ovulation detection, pair this estimate with LH surge test strips or basal body temperature charting.

Ovulation occurs approximately 14 days before the next period (the luteal phase constant). Ovulation cycle day = cycle length − 14. Adding this to your LMP date gives the peak ovulation calendar date. The fertile window starts 5 days before ovulation (sperm can survive up to 5 days in fertile cervical mucus) and ends 1 day after ovulation (egg viability window). Next period date = LMP + cycle length. All dates are estimates based on a regular cycle; individual variation is normal. For precise ovulation detection, use LH ovulation predictor kit strips.