How it's calculated
After ovulation, the time until the next period (the luteal phase) is usually about 14 days. So ovulation is estimated by counting back 14 days from the next expected period.
fertile window = 5 days before ovulation through ovulation day
next period = first day of last period + cycle length
The fertile window is six days because sperm can survive for several days, while the egg lives for about a day. In a study of healthy women trying to conceive, pregnancies came from sex during a six-day span ending on the day of ovulation, with the chance highest near ovulation.
Worked example
Your last period started on October 1, 2026, and your cycle is 28 days. Ovulation is about October 1 + (28 − 14) = October 15. The fertile window is October 10 to October 15, and the next period is due October 29.
With a 35-day cycle, ovulation is about October 1 + 21 days = October 22, and the fertile window is October 17 to 22.
When this estimate may not apply
- Irregular cycles. If your cycle length changes by more than a few days from month to month, a calendar estimate can be well off. Ovulation tests or tracking temperature and cervical mucus give a better signal.
- Ovulation varies even in regular cycles. Research has found that the fertile window can fall on very different days of the cycle from one month to the next.
- Birth control. This is not a contraception method. Hormonal birth control stops or changes ovulation, so the dates don't apply while using it or soon after stopping.
- Breastfeeding, recent pregnancy, PCOS and perimenopause can all change ovulation timing.
Sources
- Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation. N Engl J Med. 1995;333:1517–1521 the six-day fertile window.
- Wilcox AJ, Dunson D, Baird DD. The timing of the “fertile window” in the menstrual cycle. BMJ. 2000;321:1259–1262 how much ovulation timing varies.