Ovulation calculator
Fertile window from cycle length.
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How it's calculated
The second half of the cycle, from ovulation to the next period, is the steady part at about 14 days; the first half is what varies. So ovulation is expected cycle length minus luteal length days after the period began: day 14 of a 28-day cycle, day 18 of a 32-day one. Sperm live up to five days and the egg about one, so the fertile window runs from five days before ovulation to the day after. Counting the days since your period started places today in that picture. If you conceive this cycle, the due date falls 266 days after ovulation, which is the familiar 40 weeks from the start of the period when the cycle is 28 days.
When am I most fertile?
The two days before ovulation and the day itself; conception rates are highest from intercourse on those days. The full window is about six days because sperm survive up to five days in fertile cervical mucus.
How accurate is a calendar estimate?
It is a starting point. Ovulation can move by several days with stress, illness, travel or an irregular cycle, and a calendar cannot see that. Ovulation predictor kits (which detect the LH surge 24–36 hours before), basal body temperature and cervical mucus confirm the day.
What if my cycles are irregular?
Enter the shortest and longest cycle lengths you have had in turn and plan for the whole range of windows; the fertile days will span the earliest and latest estimates. If cycles vary by more than 7–9 days, or are shorter than 21 or longer than 35, it is worth mentioning to a doctor.
Results are estimates for informational purposes only and are not financial, medical or legal advice.