Menstrual Cycle & Ovulation Calculator

The Menstrual Cycle and Ovulation Calculator predicts your next period, fertile window and most likely ovulation date based on your cycle length and last period date. The average menstrual cycle is 28 days, but cycles between 21 and 35 days are considered normal, and individual cycles can vary by 2 to 7 days month to month. This tool is useful for period planning, fertility awareness and tracking cycle patterns over time.

Ovulation typically occurs 12 to 16 days before the next expected period — not necessarily at the cycle midpoint. For a 28-day cycle, ovulation falls around day 14. For a 35-day cycle, around day 21. The fertile window spans the 5 days before ovulation plus the day of ovulation itself, because sperm survive up to 5 days in the reproductive tract while an egg remains viable for only 12 to 24 hours after release. The luteal phase (ovulation to menstruation) is relatively constant at 12 to 16 days, while the follicular phase is more variable.

Cycle tracking becomes more accurate when combined with basal body temperature (BBT) monitoring — temperature rises by 0.2 to 0.5 degrees Celsius after ovulation and stays elevated until the next period. Always consult a healthcare provider for fertility-related decisions. Use our Pregnancy Calculator for due date estimation and our Date Calculator to track cycle intervals. Free, no sign-up required.